Improve support for todo widgets

[neiljp]

Initial work in #1549 by @rsashank provided

• parsing of todo data
• rendering into a styled message
• updating of the styled message upon changes in the server (events)

This issue is intended to split out todo-related follow-up work, part of which was originally tracked in #986.

Outstanding elements include

• Enable responding to tasks, ie. toggling the 'check mark' or 'done' status
• Allow editing the title in your own todo list
• Allow adding tasks to todo lists
• Limiting the editing of todo lists in the client (widget messages (content) may not be edited)

[AngathG]

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[neiljp]

@AngathG @Shashwat-Harsh-dev This has multiple parts, so if you wish to work on different pieces, that would be fine - or we can discuss on chat.zulip.org first, if some parts are not clear :)