send users an email with a clef invite link
fix a bug where editing another user's profile was showing clef state for current user
- escape strings passed to render_template
- isset_{GET|POST} for getting a $_{GET|POST} variable if it exists
use the user's email in the invite link, so we can look it up later
Allow users to log in with password (even if they are disabled) if they have a valid invite link:
- if invite link older than 3 days, an error is shown
- successful login takes them to edit profile (for now)

Also:
- refactor determining whether passwords are disabled for readability
- use render_template instead of including template directly
Merge branch 'invite-users' into two-point-oh

Conflicts:
	includes/class.clef-login.php