Fix domain checks

[eikehartmann]

Fixes #198

• given a domain vouch.github.io as config value vouch.domains and user with email [email protected]

  • expected result: User won't be authorised.
  • actual result: User is authorised.
  • patched result: User won't be authorised.

• given a domain easy.vouch.io as config value vouch.domains and user with email [email protected]

  • expected result: User won't be authorised.
  • actual result: User is authorised.
  • patched result: User won't be authorised.

From looking at the code, domains.IsUnderManagement(user.Email) which uses domain.Matches() method will create the misbehaviour described above. The PR should fix this.