Ability to reply to email notifications

Moved from https://github.com/vector-im/element-web/issues/1572

> Two ways of doing this: an email->matrix bridge, or 'reply' links on the notifs. We already have the latter, so I assume this is the former. The bridge would not be easy though as if you have multiple notifs in the email, which are you responding to?

> A better solution would be a separate email<->matrix bridge I suspect.

Start by reading the linked Element Web issue and the discussion quoted here. The issue presents an email–Matrix bridge and reply links as alternatives but does not select an approach, identify specification files, or define completion criteria. A contributor would first need the scope and protocol requirements clarified.

Written by the indexing model from the issue text.