Parse Matrix mentions from HTML

[jonnyandrew]

Room and user mentions

When setting the content of the editor from HTML, the editor should parse links matching the Matrix permalink format as mentions.

A URL is a mention if it has the Matrix URI scheme or is a Matrix.to link:

• Matrix URI scheme: matrix:[//{authority}/]{type}/{id without sigil}[/{type}/{id without sigil}...][?{query}][#{fragment}] (see also recommended parser implementation)
• Matrix.to: https://matrix.to/#/<identifier>[/<extra parameter>][?<additional arguments>]

Apps also seem to create and accept custom matrix.to style mentions:

• Custom app mention: https://<app-host>/#/<type>/<identifier>[/<optional-event-id>][?<optional-query>]

where

• <app-host> is one of a set of supported hosts defined by the host application (e.g. app.element.io)
• <type> is one of room, user or group (see MatrixToConverter).

Whole room mentions (@room)

When setting the content of the editor from HTML, the editor should parse plain text that matches the pattern @room as a mention.

[jonnyandrew]

Most of the functionality required here should already be part of ruma (see matrix_uri.rs)

[artcodespace]

Addressed in 1bf905d