Password reset

[Terreii]

Is your feature request related to a problem? Please describe.
With #287 the functionality of resetting a lost password was removed. This must be added back.

Describe the solution you'd like
When a user signs up or changes their username (email) should a confirmation mail be send.
This email should contain a link to a confirmation URL. If this URL is opened, then that email should be able to reset a password.

On the password reset:
A mail should be send with a link to a reset page. On that page the user can enter one of the reset-keys to get access to their data and choose a new password. If the user doesn't have their reset-keys, then their data will be lost.

Describe alternatives you've considered

• Maybe look into Mozilla's sync protocol again.
• Don't encrypt the local data, but only when the data is synced?

Additional context
The reset-keys are those from hoodie-plugin-crypto-store.