Add basic threading support (without prefix context)

[hloeung]

This adds basic and lightweight threading support, which means showing parent post / thread ID when present and support for replying to thread. With this, if given a post ID (e.g. ${URL}/pl/wdtrfo13p3yozq479n3coafzpy), you can reply to older threads / posts.

|11:53 <hloeung> Testing new thread support
|11:53 <hloeung> Testing reply to thread (re @hloeung (@@fkd7r1hhypgyzmaztrwuerf4pr))
|11:53 <hloeung> @@fkd7r1hhypgyzmaztrwuerf4pr Testing reply from IRC ^ last one was from web UI
|11:53 <hloeung> @@fkd7r1hhypgyzmaztrwuerf4ps Testing invalid parent post

[42wim]

Sorry for the delayed response, I'm not sure yet about merging this as we need to support two types of threading this way :) I can understand support of replying to older threads, but what do you think about using suffixcontext when showing new/current threads ?

[hloeung]

Sorry for the delayed response, I'm not sure yet about merging this as we need to support two types of threading this way :) I can understand support of replying to older threads, but what do you think about using suffixcontext when showing new/current threads ?

No problems at all.

It's a more simple approach to threading showing the actual post list ID. Also lets you bring up said thread or point via the Mattermost UI as well as easier searching through IRC logs. But sure, understand that PrefixContext/SuffixContext exists for threading support so might not be best to support another type.

Happy to close / reject this PR. Thanks.

[42wim]

I think the support for replying to older mattermost threads is very useful. If you're still interested I would accept a PR to support this part of your changes.