Get/GetKey/SetKey/DeleteKey methods are used for access to
email/discord/telegram/matrix, everywhere. Mutex added for each, avoids
concurrent read/write issues. Will also make potential transition to
database easier.
"Require unique ..." Settings (`require_unique` in relevant sections of
config.ini) are now available for email/discord/telegram/matrix. An
error is shown on the invite form if a non-unique address/ID is used.
This was on my kanban without a link to an issue, so i'm guessing it was
requested on Discord.
checkInvite no longer loops over all invites and checks for expiry, that
functionality has moved to checkInvites. Couple more rogue print
statements removed aswell.