Connecting to the `soledad_bootstrap_finished` signal is not enough
since the password change window is created after the signal is emitted,
that way we were not able to tell when soledad is ready to be used.

- Resolves: #7474

these methods were returning None, and therefore breaking soledad
password change (since it checks whether mail is enabled before changing
soledad pass after srp pass change).

- Resolves: #7470

Right after a complete sync we show the unread emails.

- Resolves: #7453

- Resolves: #7415

Look for file defined in the `BITMASK_CREDENTIALS` env variable and load
`provider`, `username` and `password` data.
Trigger login if they were loaded correctly.

The credentials file should look like this:

[Credentials]
username = my-account@my-provider.com
password = secret

- Resolves: #7419

If the user wants to register a new account we check whether the
provider allows registration or not right after getting the
provider.json file and show an error msg on the wizard if not allowed.

Also, there is a new signal to handle the error raised by the server if
a registration attempt is made but is rejected with error 403.

- Resolves: #6594