ui: optional announcement

[mvidalgarcia]

This can be useful for reana-dev and reana-qa instances or in case we want to announce e.g. an intervention with expected downtime.

Quick draft idea:

[tiborsimko]

Also, on QA, we could display some permanent warning text there:

(!) Please note you are using REANA QA developer preview site. If you are looking for REANA production site, please go to _reana.cern.ch_.

[mvidalgarcia]

I was wondering what is the most effective way of implementing this. Some ideas:

• a) Easier way: Just a simple key in config.js that prints its content if exists. Probably not ideal in terms of enabling the announcement on-demand as it'd require rebuilding the image.
• b) Harder way: And env var on r-server whose content is sent when user is loaded (/api/me) and email, token, etc. are saved in the redux state.

I invite you to share some other ideas you might have or express your preferences.