Add basic `m.thread` support

[greentore]

Description

Adds basic m.thread support as described in MSC3440. Support in this case means that replying to a threaded message will place the reply in the thread in clients that support them.

Partially fixes #257

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings

[github-actions[bot]]

Preview: https://1349--pr-cinny.netlify.app ⚠️ Exercise caution. Use test accounts. ⚠️

[naskya]

any updates?

[filipesmedeiros]

Is any help needed here? I can try to contribute if you think I could help :)

[JuliaSoriaSmith]

Can the reviewers with write access please look at this? This has been open for quite some time and threads are a noticeable missing feature

[greentore]

I added threaded reply indicators inspired by element-x. Should resolve https://github.com/cinnyapp/cinny/issues/1871.

[williamkray]

i'm not an approver but i've briefly tested the netlify deployment of this PR and it looks fantastic, i'm very eager to see this merged.

seems like the quoted message doesn't get truncated with elipses (...) properly, and runs off the right hand side of the screen.

[williamkray]

same messages in stable cinny:

[greentore]

@williamkray looks like I broke it again. Thanks for the report.

[williamkray]

looks good after the fix!

[greentore]

I converted rem units to equivalent toRem calls as requested. I also considered disabling thread support in sdk, but resulting unthreaded read receipts mark all threads in the room as read in threaded clients, which may be considered undesired behavior.