Multiple e-mail addresses

Add multiple e-mail addresses to accounts and select one as the primary and the recovery address.

Fixes #62 (closed), first step towards #80 (closed)

Selfservice:

Admin interface:

Follow ups:

• Services integration of multiple e-mail addresses
  • Allow users to choose which verified address to use for a service (#80 (closed))
  • Allow users to disable emails for a service
  • Allow admins to completly disable emails for a service
  • Integration for services that themselves support multiple email addresses (e.g. Mailman3 and Gitlab): Provide them all verified email addresses and link to their mail preferences in uffd
• Optional unique constraints (#145 (closed))
  • Decide on a key function for e-mail addresses (case-normalization, utf8-normalization, space-normalization?)
  • Enforcing e-mail address uniqueness globally is a breaking change, so we need to find a way to make this optional