1092 map loading background

Closes #1092.
Partial progress towards #907 The spinner inside the loading modal in home.clj is untouched by this commit, but should be modified as well.
Partial progress towards #1100 (just that maps don't load properly if you load `/table` and `/chart` views so far). While doing this, I removed `view-for-context`, which was previously used to generate a different view for each of the map, chart, table pages (all of that is done in (cl)js now). Only server-side dataview now is the single chart view (which will become the embedded chart view).

I also added `ona.utils.shared-dom` which has a `loading-spinner`, which is used in `ona.viewer.templates` in the `inline-loading-spinner` and `fullpage-loading-spinner` templates.