add a configurable banner that can be displayed during the TEI migration and then turned off

[rlskoeser]

hopefully we can adapt from the test banner we used to have and use on other sites; should make the text configurable (or maybe overridable with default test banner language), and should be able to enable/disable using a feature flags approach in django settings

[rlskoeser]

Tested in qa and it works beautifully — enabled it; revised the text; disabled it.

Worth noting, it does require an nginx reload before the changes take effect.

@blms I forgot to discuss with you, is the warning banner also included in our admin base template? And if not, how much trouble to add it?

[rlskoeser]

confirmed working in django admin as well