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

Refactor ivan's code (aa4b684d0682ff9faf1577653fa5ceabbc6e0f20) to remove the
time.sleep.

- Resolves: #7414

the second callback was actually broken, not being called when it was
supposed to be.
the reason is that IncomingMail.startService returns a deferred which
callback is called with the loopingCall instance only when the
loopingCall is stopped.

logout calls were not being authenticated, so we were receiving 401
return code.

- 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