Feature: Add an unarchive dashboard button

[kyoshidajp]

Fix #982.

It would be easy to restore without directly updating DB ☺️

Screenshot

Replaced "archived" label to "Unarchive" button.

Confirm modal:

[kyoshidajp]

Moved an unarchive button to the button group.

Actually, other buttons are not displayed when a dashboard is archived. So, I changed that only unarchive button is shown.

[kyoshidajp]

@arikfr Could you review this? I hope this feature helps many users.

[arikfr]

Hi,

(This is a template message, but I mean every word of it. Also you're welcome to reply)

Thank you for making this contribution. While we couldn't bring it to completion and merge, it's still very much appreciated. 🙇

In the past year the Redash code base gone under massive updates: on the backend we moved to Python 3 & RQ instead of Celery and on the frontend we replaced Angular with React. It's very likely this makes this PR irrelevant without significant changes. :-(

I'm closing this PR now. But if you're still interested in making it happen, let me know and I will reopen.

Thanks.