Subdirectory Hosting Support + Security Fixes + Misc Improvements

[AndrewPaglusch]

This PR:

• BIG CHANGE Adds a pre-flight "sanity check" to help admins troubleshoot common installation errors
• BIG CHANGE Enables FlashPaper to work when hosted from a sub-directory (ex: /flashpaper instead of /).
• Catches possible errors when encrypting/decrypting secrets
• Catches multiple possible database errors
• Fixes a problem with the footer of the page being displayed incorrectly on small screens (phones)
• Updates Bootstrap from 4.0.0 -> 4.4.1
• Adds more reliable detection and warning if FlashPaper is accessed over HTTP and not HTTPS (client-side as opposed to server-side checks)

[Matthew-Jenkins]

Need to add vacuum after delete to make sure data is actually purged from sqlite.