Refactor the Slack Application integration form: Part 1

What does this MR do and why?

The MR refactors the shared integration form to separate the integration help text from the "fields without sections". The former (the help text) should always be at the top. The latter should go at the bottom of the form.

Why do we need to change it now?

The reason the issue is not visible is that once an integration gets sections (for example, Jira) the current implementation hides the help text, assuming it should be one of the sections then. Hence the location of this block on the page doesn't matter. It used to work fine: we either show the integration help text and no sections, or we show sections and, if needed, help will be one of the sections.

However, now, when working with Add notifications to Slack App integration - Part 1, we will have a situation where we might have both: the help text (which won't necessarily come from the sections) and the sections on the same form. Hence we have to make sure the help text always resides on top of the sections and not below those.

Screenshots or screen recordings

No visual changes for now:

	Before	After
Slack Application
Jira

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #375345 (closed)