This change solves the User's API problem when you want to create an user that its email hasn't been verified yet but it exist.
At now, you can resend invitation email using endpoint /users/{id}/resend-invitation
Fixes#184
The DB implementation expects secrets to be base64 encoded blobs.
Because of this a bunch of tests broke moving to sqlite.
A lot of this commit is fixing those tests.
Invite emails are essentially just reset password emails with a
different template (though this can and probably will change (slightly)
in the near future)
