Removed submission preference alert from top of config page

[zenmonkeykstop]

Status

Ready for review / Work in progress

Description of Changes

Fixes #5220

Adds document submission flash message to the list of messages not displayed at the top of pages, similarly to the handling for the logo form on the instance config page. (It was being displayed unstyled due to the message category not having a corresponding css style)

Testing

• checkout this branch and run make dev
• log into the journalist interface using the test credentials, and navigate to the config section: /admin/config
• change the submissions preference at the bottom of the page
• the config page is re-rendered without a flash message at the top of the page, and with the "Preferences saved" message rendered correctly in the Submissions Preferences section.

Deployment

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

[rmol]

I thought we were going to make it only appear at the top of the page?

[zenmonkeykstop]

@rmol IMO there's a good case for some UX attention on the admin interface in general, including alert placement. But for now this is just a bug fix to restore the intent of the original PR.

[rmol]

👍 Sounds good, looks good, works good.