Optimade widget polishing for QeApp

Nicola what to have following feature added/fixed for the optimade widget, which will make OPTIMADE widegt more user friendly and move obvious to use in QeApp as the most popular structure importer.

- [ ] The "Select provider" and "Apply filter" harder need to be more bigger so users know what the current step is and what they are supposed to do next.
- [ ] The OPTIMADE selection module steps should be numbered (e.g. "1. Select a provider" and so on).
- [x] ~~Add a "rocking message" that shows "Select provider" then changes to "Loading provider…" to the 2nd step.~~ Not needed if the loading is fast.
- [ ] When database is chosen, the first query for max number of sites and elements caching moved from class to file and make a static file which will only be updated the next time deployed. (https://github.com/CasperWA/voila-optimade-client/pull/446)
- [x] #288
- [x] #287
- [x] #286
- [ ] The button to download all queried structures. https://github.com/CasperWA/voila-optimade-client/issues/233