Password validation added, configurable under the [password_validation]
section in config.ini. Each criterion is displayed next to the form, in
red or green depending on whether the password passes it. form.html now
looks different because of it, whether it is enabled or disabled. An
error message is now displayed if the user already exists.
Changed admin.js so that instead of deleting all invites and then
generating them, new ones are created, pre-existing ones have their
expiry updated, and newly invalid ones are deleted. Accidentally
improved their layout, as well.
