Improved conversation message attachment layout

Partners can now link an existing client to an existing site through the Public API.

Conversation attachments now use a more compact layout, making messages with many files easier to scan.

Transcript and audio attachments use more of the available width on mobile for easier reading and playback.

Fixed edge case where users trying to set assignee would cause application to flicker on Windows devices.

Editing a recurring Signal schedule no longer creates duplicate or missing events after save.