WIP: Self-Signup and Invite-Links

Closed in favor of !19 (merged) and a soon-to-come MR for invite links

• generic sendmail() function
• signup blueprint with an generic self-signup mechanism (must be enabled in the config)
• invite blueprint with an admin interface for invite links for role assignment and account creation

Signup:

• Basic functionality
• User Interface
• Rate-Limiting
• Tests
• CAPTCHAs won't be implemented
• Make account creation and setting the passwort atomic (probably after !14 (merged) is merged)
• Documentation
• Change templates according to #44 (closed)
• Rename templates according to #47 (closed)

Invites:

• Basic functionality
• User Interface
• Tests
• Rate-Limiting?
• Documentation
• Minimize race conditions (LDAP user creation vs. invalidation of invite link in DB)
• Change templates according to #44 (closed)
• Rename templates according to #47 (closed)

See also #39 (closed)