Add ability to go back to a thread from the plugin RHS

[crspeller]

We want to be able to go back to a previously open thread from the plugin RHS. This is currently not possible as the back button system only supports things like search and pinned posts.

RHS code: https://github.com/mattermost/mattermost/blob/86689b7ee69b6f914f7ceacd29b2f356a6b1888c/webapp/channels/src/components/rhs_header_post/rhs_header_post.tsx#L49

Where it's used in figma designs

[jespino]

After some investigation makes a lot of sense to just take advantage of the fact that we can pass a plugin as the title and use that component to add the back button whenever we consider is needed. and track inside the plugin where to come back. I think we don't really need plugin API changes for this. I'll create an implementation of that directly in the plugin.

[crspeller]

That works as well. If the implementation is not clean we should probably fix the system. Otherwise if its simple then we can just do that direct plugin implementation.

[jespino]

I think the implementation is clean enough to do it in that way, and it looks like a preliminary step to get the whole RHS working as we want as part of the AI plugin. But I think we are going to face other kind of challenges there first. So I think this should be keep on hold. We can make it work, so no action is needed here for now. Whenever we start figuring out the whole RHS probably we have a better understanding of how deep we need to go with the extensibility in mattermost.

[crspeller]

Dropping this for now.