User registration

User registration should be available as an optional feature (with a site preference disabled by default).

It should allow the following:

• Simple registration of a pure user account (!413 (merged))
• Verification of provided e-mail address (!413 (merged) )
• Linking an existing person, if one exists with the validated e-mail address (optional, additional preference) (see #332 (closed))
• Creating a new person if none exist (see #332 (closed))

Edit: Some things moved to #453 (closed)

If an existing library is used, it must allow hooking into registration, to create accounts in linked backends (like in AlekSIS-App-POSIX/AlekSIS-App-LDAP).