Subdirectory Hosting Support + Security Fixes + Misc Improvements

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)

AndrewPaglusch / FlashPaper