replaced ajax for compatibility with jQuery slim

[VincentTam]

Edited: I prefer merging #782

Goal

To resolve #766

Background

Brief evolution history of the part of code concerned for Staticman integration over the past year:

1. I ported @mmistakes's code in #440 and #444 for Staticman integration (in response to #115).
   

   beautiful-jekyll/_includes/staticman-script.html

   Lines 4 to 6 in 3a45190

   	if (typeof jQuery == 'undefined') {
   	document.write('<script src="{{ site.baseurl }}/js/jquery-1.11.2.min.js"></scr' + 'ipt>');
   	}

2. I moved the JS code in Staticman integration from the Jekyll-HTML template file into a JS file in #521, so that the JS code is found under a more logical location.
3. The jQuery imported in item 1 was replaced by its slim version at 5991966#diff-e9c677c6d71b6acd8eb0963e0e2a9bd997a86240a3711c2d391c42d18eb12c32.

Description

The ajax method used by jQuery can be replaced by the fetch method, which belongs to standard JS. In this way, the problem would be gone.

Testing

To properly test this PR, you need a working Staticman API instance. I've inactivated mine @staticmanlab due to Staticman's maintainer's advice to move away from any public API instances in the comments in eduardoboucas/staticman#337. For a Staticman API instance setup guide, you may see, for example, what I've written in #775, or https://github.com/pacollins/hugo-future-imperfect-slim/wiki/staticman.yml, which are inspired by Jan Hajek's article about Staticman v3.

1. I've merged the sm3slim branch into the gh-pages branch for my fork of this theme:

   VincentTam/beautiful-jekyll@e37a4fd...VincentTam:bccdcafb.

   P.S. I had made a careless mistake in the endpoint site config variable while merging the recent development into my gh-pages branch. I corrected that at bccdcaf.

2. Make sure that you've correctly filled in the theme-specific site config. Here's the one used in this test.

   # To use Staticman comments, uncomment the following section. You may leave the reCaptcha
   # section commented if you aren't using reCaptcha for spam protection. 
   # Using Staticman requires advanced knowledge, please consult 
   # https://github.com/eduardoboucas/staticman/ and https://staticman.net/ for further 
   # instructions. For any support with staticman please direct questions to staticman and 
   # not to BeautifulJekyll.
   staticman:
     repository : vincenttam/beautiful-jekyll # GitHub username/repository eg. "daattali/beautiful-jekyll"
     branch     : gh-pages # If you're not using `master` branch, then you also need to update the `branch` parameter in `staticman.yml`
     endpoint   : https://secretman3.herokuapp.com/v3/entry/github/ # URL of your deployment, with a trailing slash eg. "https://<your-api>/v3/entry/github/"
   #  reCaptcha:   # (optional, set these parameters in `staticman.yml` as well) 
   #    siteKey  : # You need to apply for a site key on Google
   #    secret   : # Encrypt your password by going to https://<your-own-api>/v3/encrypt/<your-site-secret>

3. I sent a sample message through my online form on my testing site : https://git.io/bjsm18.

   

   fields	value
   fields[message]	new post method
   fields[name	Vincent Tam
   fields[email]	xxxx@live.hk
   fields[url]	https://vincentkltam.gitlab.io
   options[origin]	https://vincenttam.github.io/beautiful-jekyll/2020-02-28-test-markdown/
   options[slug]	test-markdown

4. I observed a success message. My Staticman API's (secretman3 d0τ herokuapp d0t com) response:

   {
     "success":true,
     "fields":{
       "message":"new post method",
       "name":"Vincent Tam",
       "email":"4980f828b9588a712cc2013ac10fd13a",
       "url":"https://vincentkltam.gitlab.io",
       "hidden":"",
       "date":"2021-02-09T12:41:42.498Z"
     }
   }

5. Successful PR: New comment by Vincent Tam VincentTam/beautiful-jekyll#35

P.S. I made error in the endpoint variable at VincentTam@f90ee57#diff-ecec67b0e1d7e17a83587c6d27b6baaaa133f42482b07bd3685c77f34b62d883R183. That enabled me to view the displayed error message after merging the sm3slim branch at 36e8bca before correct that variable at the following commit.

[VincentTam]

Closed this in favor of my newer PR created in response to the discussions with the author of linked issue.

[daattali]

Thanks Vincent. I'm swamped with work on other projects but will come back to this, I appreciate it