Improve help text for Maildir folder

[jugmac00]

When your Queue directory is misconfigured, you see two messages in ZMI.

1. This must be a Maildir folder directly below the input field
2. n/a - /chosen/path/to/mailhost is not a maildir - please verify your configuration at the bottom of the page

I'd like to improve the wording and maybe also the exception handling, as the message is not really clear and possibly suggests you would have to know how to create a valid Maildir folder (with the typical subfolders yourself.

The new text should mention that the directory gets created automatically (as far as I remember) as long as you choose an existing path but a name for a directory which not exists.
Ie when /srv/spool exists you can enter /srv/spool/maildir but not /srv/spool/a/maildir.

If nobody pushes to the front, I'd like to go for it - in the new year, though.

Opinions?

[dataflake]

I am in the code right now and will incorporate several improvements to that page. As I want to create a MailHost release I will close this issue afterwards. Please reopen or create a new issue if there's still a problem.

[jugmac00]

With @dataflake 's changes applied, and when you enter an invalid directory path and hit "Save Changes"...

• you get a confirmation message with green background (suggests success) at the top; also see screenshot
  

• no more warning at the info box for "Queue directory"

• at the bottom of the page you still see n/a - /chosen/path/to/mailhost is not a maildir - please verify your configuration

Some thoughts:

• either prevent "deep" directories (where also parent directories have to be created, e.g. if /var/spool/ exists allow /var/spool/maildir but not /var/spool/parent/maildir which results in n/a - /chosen/path/to/mailhost is not a maildir - please verify your configuration) or update the underlying code to create "deep" directories similar to mkdir -p
• depending on above bullet point, adjust the current message "otherwise the directory will be created for you" - as it only gets created if the parent directory exists
• do not allow to set an invalid path at all - and show an error message instead of a green "success" message
• or at least highlight the current "warning" n/a - /chosen/path/to/mailhost is not a maildir - please verify your configuration
• provide a link from "Maildir" e.g. to Wikipedia https://en.wikipedia.org/wiki/Maildir