Build of "Hacks" area on the site

[ssddanbrown]

This PR is for the initial creation of a "Hacks" area of the site to list customizations.

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Todo

• [x] - Include warning/advisory about non-supported status and potential to break things.
• [x] - Finish off base implementation.
• [x] - Test with each type of modification: Custom head, Logical theme, visual theme.
• [x] - Cleanup sidebar
• [x] - Format /hacks list page.
• [x] - Add link to hacks from header, footer and "hacking bookstack" page.
• [x] - Move hacks content to their own repo and have as a submodule?
• [x] - Copy over other hacks from past gists.
• [x] - Update site server to provide PHP/JS/CSS files as plaintext. And *.html requests? (Could scope to hacks path to be safer?)