Add option to subscribe to and pin reply threads

[Nerixyz]

Description

There are two changes in this PR. First, the option to change the subscription status of a reply thread is added. Secondly, a pin-button is added, like on user cards:

This adds the concept of a subscribed thread. If one is subscribed to a thread, new messages will be highlighted (requires the Subscribed Reply Threads highlight option). One can subscribe to threads through the popup and by participating in a thread (requires Subscribe to participated reply threads).

[github-actions]

clang-tidy made some suggestions

[Nerixyz]

Regarding #4678:

C:\Users\runneradmin\.cargo\bin\sccache.exe -s
  Compile requests                    474
  Compile requests executed           474
  Cache hits                           67
  Cache hits (C/C++)                   67
  Cache misses                        407
  Cache misses (C/C++)                407

I think the git output is causing the misses, since it adds different definitions every time - 67 hits are from the crashpad_handler.

[Felanbird]

Seems to work exactly as I would expect it to, pin properly only shows up when it's related auto hide setting is enabled, same with the subscribe feature only appearing if you have thread highlights enabled. 👍