Activation + Reset token emails

There's already a reset token mechanism available by mix task. This needs to be exposed in an API.

Things to do

• add an SMTP library
• Add an option for users to invite other users (configurable in the config)
• Add the frontend part for this to PleromaFE (for Admins and Users) — to be implemented in pleroma-fe#213 (closed)

It would also be nice to have an option to require an 'activation' email before a user can start using the site.