To keep compatibility with soledad upgrades. It will namely cause version conflicts
to not be properly handled otherwise.

One of the pixelated adaptors was trying to access a non-existing
attribute in HashableMailbox, which for some reason was blocking the
operation of the imap server (uncatched exception in listeners call
maybe).

adding an attribute skips this error and therefore allows seamless use
of both pixelated and thunderbird user agents at the same time.

Resolves: #8083

for consistency with the previous Account change.

Previously Account used user id from the store, but this attribute is
optional and None by default. This caused the collection_mapping to be
unable to distinct between multiple users message collections.

This chance adds a non optional user_id attribute to Account and use it
to index the collection_mapping.

- Resolves: https://github.com/pixelated/pixelated-user-agent/issues/674
- Releases: 0.4.0

This greatly simplifies the handling of the password in the thunderbird
extension.

Related: #6041

this was gone with the imap/cred refactor, but the client relies on it
to hide the 'congratulations!' welcome display on the mail widget.

- Resolves: #7898