Improve visibility and management of Input static fields

## What?

Input static fields should be easily visible, and their management intuitive.

Specifically:
1. Static fields should be shown on the Input Diagnosis page, as it's likely a user will end up there (by clicking on the Input name link).
2. The `Static fields` section of the "expanded Input row" content should always be displayed, even if there aren't any static fields currently set.
3. `Edit input` should be the sole location to manage an Input's static fields.
* Bonus points: it should be possible to edit an existing static field entry.

## Why?

Prior to `7.1` Input static fields were visible below each Input, and it was reasonably clear how to add/remove them.

As of `7.1` (specifically the new Inputs page):
* The only way to see an Input's existing static fields is to click the Input's row-- but importantly, _not_ the Input name link since that goes to the Input Diagnosis page which _does not_ show the Input's static fields.
* I'd expect clicking the Input name link to result in a more detailed view of an Input, and clicking the rest of the row would show a "quick view" of the Input config; it's unexpected that the latter would have information not present in the former.
* There are two ways to add a static field to an Input:
1. `More --> Add static field`
2. (_Only_ if there are existing static fields, since the section doesn't appear otherwise) Expand a row and click `Add static field`
* The only way to remove a static field from an Input (in the UI) is to know that you have to click the row (but not the Input name) to get to the `Static fields` section that allows removal.

Related: #24653

## Your Environment

* Graylog Version: `7.1.2` (Enterprise)