Added an unlinking section to the little cog dropdown next to users so
that one can remove and re-link a different account for a Jellyfin user.
Also adjusted padding in the dropdown.
"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.