Improve facet loading message
Move `nwbibsubject` and `nwbibspatial` into facets (see #78)

- Remove from advanced search preselection, replace with `subject`
- Add two new facets `nwbibsubject` and `nwbibspatial`
- Rework UI concept for facets: move elements from top bar to side
- Group side UI elements under explanatory headers
- Highlight selected facets to make current restriction obvious
Restrict title autosuggest in advanced search to NWBib set

Thanks for reporting the issue @donboern and @dr0i
Tweak labels in faceted search (see #78)