Standardize confirmation/error message placement in Admin Interface

[eloquence]

The web-based Admin Interface allows admins to send test OSSEC alerts, update a logo, and update submission preferences. In #5046 we added a confirmation message after changing the submission preferences, and in #5223 we removed duplicated display of that message.

The confirmation message for submission preferences is now displayed at the bottom of the page, where it is easy to miss.

We should standardize the confirmation/error message display in the Admin Interface, with an eye to ensuring that confirmation messages are clear and noticeable. Solutions could include:

• always anchoring to the relevant section to ensure that the message is in view
• always displaying confirmation/error messages at the top of the page

It's worth noting that the OSSEC test alert is not really a setting. It may make sense to organize the page(s) differently, which could influence how we want to handle confirmation/error message display.

Screenshots of current behavior

Test alert

(Note how this confirmation message is displayed above even the "Back to admin interface" link)

Logo change

Logo error

Submission preferences change