The admin page now has the option to send an invite to an email address.
Since there are now two email types (invites and pw resets), the new
sections have been added to config.ini, and email_template and
email_plaintext have been renamed to email_html and email_text
respectively.
Invites and the generate invite form are now displayed as cards. Invite
codes are displayed in a monospace font so they are always aligned with
eachother. The delete button is now inline with the rest of the text.
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.
