Add a customizable banner for admin messages

[sbliven]

Is your feature request related to a problem? Please describe.
It can be important to inform users about service disruptions. It would be nice to have a way to show a banner to users without editing source code

Describe the solution you'd like
There should be a way to specify a simple banner message, either as a config file or with an environmental variable. For instance:

LANDING_PAGE_BANNER_MESSAGE="Retrieving datasets is experiencing intermittent issues. For details see https://status.psi.ch"
LANDING_PAGE_BANNER_COLOR="red"
LANDING_PAGE_BANNER_LOGO="triangle-exclamation"

This would add a small dismissable banner at the top of the page.

It should be easy to update the banner without recompiling the landing page or rebuilding the container image.

Describe alternatives you've considered

Additional context

[omkar-ethz]

paulscherrerinstitute/scicat-ci#268

[minottic]

this is closed, right @omkar-ethz ?

[omkar-ethz]

this is closed, right @omkar-ethz ?

yes exactly, thanks! closed by #394