theme: add configurable banners similar to INSPIRE

[GraemeWatt]

The inspirehep.net site has a feature where a BANNERS variable can be added to the Kubernetes configuration with some options. The banner messages are then displayed at the top of the inspirehep.net pages. It would be nice to have something similar for the HEPData site, where the BANNERS variable would be defined either in config.py of the web app or in hepdata.cfg of the Kubernetes configuration. This would allow temporary messages to be displayed, for example, in the event of planned interventions, without having to modify the hepdata/hepdata Docker image. These temporary messages should be displayed on all HEPData pages, since a user might not navigate the HEPData site starting from the homepage. At the moment, we can post Twitter messages, but these tweets can often be missed by users not following @HEPData on Twitter or arriving at a HEPData record directly rather than going via the homepage.