Channel search

Once a user is joined in multiple channels, the selection list can become very long. An input for a search term would be nice in order to find the right channel.

## Acceptance criteria
_tbd_

Start by locating the channel selection list in the Outlook plugin and trace how joined channels are loaded and displayed. Define the expected search behavior with the maintainers, then verify that entering a term narrows the list to matching joined channels without disrupting selection.

Written by the indexing model from the issue text.