Moderator authentication

[mushrom]

Need to have a login for mods, and a way to register new users.

Flask has built-in support for cookie encryption, so we can just use that, and for authentication I'm thinking a table which just has admin IDs and what their access levels are.

[notgne2]

Per board would be really useful but not absolutely necessary, storing a list of the moderators' actions in the DB would be nice for the future too.