Feature: allow slack messages to be updated

[DaanMeijer]

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Description

Fixes #3898
Implement the Slack App way of posting messages, as well as allow for the updating of previous messages. Features a complete restructure of the way Slack messages get created

Type of change

• User interface (UI)
• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings

Screenshots (if any)

[louislam]

Please note that some changes may be duplicate of #3394.

[DaanMeijer]

Please note that some changes may be duplicate of #3394.

I could remove the part regarding duration. It's not necessary for this functionality, but I thought it would be a nice bonus (and it takes into account the possibility of the duration being unknown, because of the limitations with heartbeats being stored for a limited time).

[DaanMeijer]

What's also worth noting, is that this pull request is based on #3886, as it expands on the implemented functionality in that pull request.

[CommanderStorm]

Please note that some changes may be duplicate of #3394.

I could remove the part regarding duration. It's not necessary for this functionality, but I thought it would be a nice bonus

Sorry for the late review. After rereading the content, the messaging here was not clear.

=> Please remove the changes regarding the duration.
This feature should be handled at a higher level and not just in this monitor, making future work more difficult

Also note that there are a lot of changes in this PR. I don't think most are intentional => please remove those who are not

[DaanMeijer]

@CommanderStorm I've restructured the code for this pull request to be more legible. There is a lot of code, which actually is necessary, because this feature requires a different method of authenticating with slack.

[matttheus]

What is the status of this PR?