feat: Category specific channels

[Jake-Shadle]

This adds the ability to turn on category specific channels, which
can be specified in the category settings when the setting is enabled.

If the category does not have a specific channel set, it falls back to
the parent category if there is one, otherwise it just uses the global
channel specified in the settings.

[bernd]

Thanks for the PR! I just asked in #6 if this would cover the use case.

[Jake-Shadle]

Cool, let me know if you want me to clean anything up, I am a complete n00b at Ruby so it's probably not idiomatic. 😞

[richardxia]

This looks pretty useful! However, it'd be nice if we could have the ability to set SiteSetting.slack_channel to None to prevent messages from being posted to a default channel unless if they have access a specific category. I think that this could be done after line 48 with a single line:

next if channel.empty?

I'm not familiar with Discourse plugins, but that should work if it's built on Ruby on Rails. Otherwise, you'll want to do next if !channel or channel == ''.

[Jake-Shadle]

I was actually thinking about this, except by having a checkbox in the UI so it is more obvious to the user. (would also disable text input where you put the channel/user name)

[deargle]

FWIW I'm using this PR with @richardxia's next if channel.empty? suggestion with great success.

I also added the post category name to the slack post title, but my use case is kind of specific: I'm only sending a subset of my categories to one channel (for example I'm excluding #Staff), so it's helpful for me to both be able to per-category things and leave the default category blank, and to also show the category name in the title. As @richardxia commented here I should check if category is nil to make my edit more generalizable. Maybe it'll help someone else to see what I did.

[Jake-Shadle]

Yah, I don't remember why, this was made a long time ago, put it back how it was. 😃

[deargle]

Cool thanks!

@bernd this PR has reached critical mass. It simply must be merged. (Please.) I'll write a readme PR to describe the three-ish new features in here if this gets merged.

[bernd]

@deargle Awesome, thank you!