Feature request: spatial facet keyboard accessibility

I wanted to start a conversation about adding keyboard accessibility to the _Filter by location_ facet. We're currently working with a government client with a WCAG AA compliance requirement and so we've been asked to make the spatial facet keyboard accessible. I guess governments with similar accessibility requirements form a reasonable portion of CKAN's market so this might be valuable as a core feature of ckanext-spatial.

[Keyboard accessibility](https://www.w3.org/WAI/WCAG20/quickref/#keyboard-operation) is a WCAG Level A requirement. Currently, it is possible to zoom the map and it is also possible to pan, although it is more-or-less a hidden feature. It is not possible to define a spatial extent to apply (or at least, it's not obvious and I can't work out how to do it).

After some discussion here, we've come up with the following proposal:
1. Add an explicit keyboard accessible pan
2. Add text fields for defining an extent bounding box using lat and long to the expanded view.
3. Add a button to define the extent based on the current map view --- helps user to define an initial extent when they don't know the lat and long
4. The map extent and bounding box fields are synchronised, i.e., when one is changed the other is updated --- provides visual feedback on lat and long entered
5. When no extent is selected, replace the map with a link/button labelled like 'Select area of interest' which is formatted consistent with other facets --- provides clear instruction on how to start --- when clicked shows popup with expanded map and bounding box fields, similar to current expanded view.
6. When an extent is selected, a non-interactive map is shown in the facet sidebar, with formatting to match other facets more closely

That's a little hard to visualise; I'll try to get some mockups done.

I'm also aware that @florianm has been working on some enhanced extent input widgets (#11). I don't think this work would touch on what he's doing but pinging him just in case. Also extent input widgets with greater precision will be in our future also and it will be best if those are accessible too.