This fixes two edge cases for Gmail
- When policy was set to `SELECTED_FOLDERS` excluding root INBOX, it
missed label changes, so messages with newly applied labels weren't
imported until a full resync.
- Gmail thread replies by default by default do no inherit parent
message's label properties so thread context was also lost because only
individually labeled messages were returned, dropping earlier parts of
the conversation.
Fixed by subscribing to `labelAdded`/`labelRemoved` history events and
fetching full thread context when at least one message in a thread
carries a synced label. `ALL_FOLDERS` path is untouched.
neo773
88146c2170