Remove fallback replies & implement intentional mentions

[nexy7574]

Description

This PR removes fallback replies from messages, and additionally adds m.mentions support to Cinny.

Fallback replies simply just aren't used anymore, and m.mentions works by iterating through the children of the composer to find where tabbed-in mentions have been used, and adds those to the mentions dictionary.

This means that cinny no longer accidentally pings anyone who was mentioned in messages that are being replied to.

Fixes #1544

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code (has been deployed in several places for a month)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

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[ajbura]

Internal mention also needed to be added in MessageEditor.