Add ability to output html and markdown formatted specifically for message content

[artcodespace]

We have discussed on multiple occasions the benefits of creating and exposing a function that would allow us to output the html and markdown from the Rust model but specifically for sending as message content.

This issue records some of the things we think it should do, noting that some of these approaches may change depending on other design decisions:
HTML output (to attach to formatted body)

• remove empty <p> tags from the message (ie <p>&nbsp;</p>), which we currently use to represent an empty paragraph that the user may potentially put their cursor inside whilst editing Add functions to get content in clean message format #686
• strip any attributes from <a> tags that aren't href (to remove custom attributes used for display) Amend mention message html output #716
• replace at-room mentions with the plain text @room as this is what the timeline seems to expect Amend mention message html output #716
• replace the inner text of a room mention link with the mxId (think this is what the spec dictates, should this be the same for users?)

Markdown output (to attach to body)

• replace mention link representations with just the mxId, prefixed with @ or # as appropriate

There may be more work required, but this is what we've come across so far.

### Tasks
- [x] https://github.com/matrix-org/matrix-rich-text-editor/pull/686
- [ ] Expose new get_content_as_message_html() function from platform libraries

[artcodespace]

Adding further notes here to consolidate what we've discussed in various places.

Googledoc containing points and discussion, discussed 23 May.

Summary of discussion and decisions:

• there are two separate strands of work in the document, the output and input side of the RTE
• this issue addresses the output issue only, ie adding the ability of the composer to output html and markdown in accordance with the Matrix specification
• it is probably a good idea to address this issue first, then use what we learn here to inform the approach to dealing with the input issue (ie handling html going into the Rust model, such as editing a message)
• Decisions
  • the functionality of the existing get_content_as* output functions will not be changed, so this should require no change to client composers
  • we will add a function that outputs the Rust state as html that meets the Matrix spec
  • we will add a function that outputs the Rust state as markdown that meets the Matrix spec
  • we will treat mentions as their own type in Rust to make handling them easier

[jonnyandrew]

we will add a function that outputs the Rust state as markdown that meets the Matrix spec

Would we need two different ways to get markdown output if mentions will not be represented as links in the Rust model?

[artcodespace]

we will add a function that outputs the Rust state as markdown that meets the Matrix spec

Would we need two different ways to get markdown output if mentions will not be represented as links in the Rust model?

Hmm. It's an interesting question. Initially I thought so, but perhaps not. It wouldn't affect web implementation I don't think but I'd be concerned about whether it would affect iOS at all.