Use ProviderConfig helper to get object from domain.
Refactor SoledadBootstrapper to backend.
Separate imap/smtp logic from conductor.
Move Mail logic to backend.
Avoid reconnect same signals several times.

We don't want to connect again the untracked signals after the wizard
was finished.
Move soledad password change to backend.

Also cleanup soledad usage in the GUI.
Move waiting logic for imap stop to the backend.

Also, improve quit and cleanup calls.
Improve wait and quit process.

Refactor logic from backend to the vpnprocess.
Replace twisted logs with logging logs.
Add changes files for #5481.
Rename backend methods for consistency.
Merge remote-tracking branch 'refs/remotes/ivan/feature/refactor-soledad-to-backend' into develop
Update username regex to support the same as webapp.

Rename password util to credentials and add a username check helper.
Move the username regexp to the credentials module.

Closes #5695.