Improve banner expiration validations and form

[jp524]

What github issue is this PR for, if any?

Resolves #5815.

What changed, and why?

Modified banners per issue description:

• Rename values in 'Expires At' column
• Add server-side and client-side validation to validate that expiry cannot be in the past
• Validate the banner actually has content (done server-side only. No built in client-side validation exists for <trix> elements)
• When a banner expires set it to inactive (I implemented this by modifying the existing Banner#expired? method. I'm open to feedback if there are better ways to implement this)

How is this tested? (please write tests!) 💖💪

Added some new model and system tests.

Screenshots please :)

New banner table

Expires at validation

Content validation

[elasticspoon]

Hey @jp524. Thanks for changes so far! Having poked around trying to make those systems tests work I think its better to just move those tests into request specs to simplify the testing.

[jp524]

Thanks for your help @elasticspoon ! I implemented the changes and found a couple of bugs along the way so it was a good exercise.
Some of the CI workflows are failing but they don't give an actual error or test that is failing. I did run the test suite locally though and it all passes.

[elasticspoon]

Nice job dealing with all the time zone stuff. Its a pain in the ass and I'm glad I didn't have to do it.

Overall, I am happy with the PR. I had some minor nits but nothing blocking.

[jp524]

Nits have been addressed! Should be good now @elasticspoon