Adds native HTML5 drag-and-drop so users can move recipient email
address chips between the To, CC, and BCC fields in the composer.
Chips dragged onto the Cc/Bcc toggle buttons auto-reveal the hidden
field and place the chip there.
* Added account selection for protocol links when multiple connected accounts are available, including mailto: links
* Added support for handling mailto: links in an already-open PWA/session instead of always opening a new tab
* Added webcal: protocol handling for calendar links
* Added account selection for webcal: links when multiple calendar-capable accounts are connected
* Added an import-or-subscribe choice for detected webcal calendars
* Added protocol handler settings for registering mail and calendar handlers and choosing the open mode
* Added service worker/session coordination for passing protocol requests between browser/PWA contexts
* Added tests and translations for the new protocol handler flows
