Combine SlackIntegration model with Integrations::GitlabSlackApplication

Summary

We have a model SlackIntegration in ee/app/models/slack_integration.rb, which is used by Integrations::GitlabSlackApplication.

It looks like its purpose is only to hold some additional attributes, so maybe we can move them into the properties of the main integration model.

The model is accessed/updated in a few places, but these can probably be changed to operate on the integration model directly.

Background

The following discussion from !62676 (merged) should be addressed:

• @toupeira started a discussion: (+1 comment)

  note: This is yet another model called SlackIntegration in ee/app/models/slack_integration.rb, different from SlackService which is being renamed to Integrations::Slack in !61957 (merged) 😅

  Not sure yet what the purpose is of this, we might want to rename it to something clearer too in a future MR.

  @.luke: Interesting! Obviously not for this MR at all - but out of interest, do you think SlackIntegration should be a "data fields" model (like Integrations::JiraTrackerData)?