[FR] Slack notifications & notification method updates

[matmair]

This PR:

• Adds a slack sending method
• Adds a mixin for adding settings conent
• Adds a central panel for global notification settings
  - [ ] Extends notification methods to include registering

Fixes #3843

[miggland]

@matmair: I just stumbled on this package again. Have used it earlier (CLI only) for Gotify. However, it may give the possibility of having general notifications, without explicitly adding every service independently..
https://github.com/caronc/apprise

[matmair]

@miggland looks cool, I created a new FR for it. While it is easy to send simple notifications I prefer to use native integrations as they allow for the use of platform-specific features (like Slack Block Kit for example).

[SchrodingersGat]

@matmair nice work this looks really good.

Do you think it also closes out #4073 ?

[matmair]

@SchrodingersGat I would have loved a simple one-click add button for setup but that would require me publishing a central app for slack (and therefore everyone trusting me not screwing with that access).

Not sure if everything Lukas envisioned is covered, I think we should let him decide. @wolflu05

[wolflu05]

@matmair Sorry, didn't get the context why you mentioned me. Or do you mean the new settings content mixin which gets added by this pr which is not included in the title? Form my side, I would preferre an interface more like the panels mixin which also adds the ability to render custom js templates directly and also render html templates via filename. I know this could be made already via some custom logic, but I think I'm not the only one which will try to add custom templates instead of html strings.

[matmair]

@wolflu05 yeah you were mentioned for that / #4073
@SchrodingersGat so this won't close that. I do not intend to extend the current PR as the whole custom rendering is not really working well with #3901 and would add another work site for the transition.

[SchrodingersGat]

@matmair nice work. A small docs PR would be great if you have a chance

[wolflu05]

Yeah, the whole Django rendering part with templates, plugins provide will be tricky in the react ui. Maybe it can be just embedded via iframes?

[matmair]

@wolflu05 there are some limitations with iframes from browsers so I am still trying to find another way.

[wolflu05]

@matmair from what limitations are you talking?

[matmair]

@wolflu05 some APIs can not be called by js code in an iframe.

[wolflu05]

@matmair And what about using dangerouslysetinnerhtml for legacy plugins, and current plugins can just provide an bundle file which has a default export which is included via runtime import() and rendered as component?

[matmair]

@wolflu05 I think we should take this conversation into #3901 or the subrepo - it does not really have anything to do with this issue.