Instance configuration flash message is unstyled

[rmol]

Description

In #5046 we added flash messages confirming configuration changes, with the submission-preferences-success category. They appear properly styled when presented via the submission_preferences_saved_flash.html template, but they're also presented at the top of the page without the logic that styles them as successes or errors, so they're shown as plain text.

Steps to Reproduce

Start the dev server and navigate to the instance config, then toggle document uploads under Submission Preferences. You should see a page like this, with Preferences saved. at the top (plain) and bottom (styled):

Expected Behavior

That the flash message would appear in one place, or at least be styled the same wherever it appears.

Actual Behavior

The message at the top of the page is forlornly unadorned, while at the bottom it's miles of style.

Comments

In #5046 @eloquence pointed out that the message could easily be missed if it only appears at the bottom of the page, so we should probably just make sure that the same styling is applied to both occurrences.