[#63] Preview ArcGIS online maps

Use the embed option on the remote site, as embedding the page directly
did not work.
The viewer forArgis map services can not be embed because maps need to
be saved on the remote site.
Update the descriptions of arcgis based formats (needs reindex).
[#63] Advanced map viewer page

Adds a dedicated page for the advanced map viewer, which embeds the GMU
viewer but laoding it on the advanced mode. A link to it has been added
to the resource page for those formats that can be previewed.
[#66] Adds a load of accessibility fixes
Merge branch '63-map-viewer-enhancements'

Conflicts:
	ckanext/geodatagov/plugins.py
Merge branch '66-accessibility-fixes'