Geonode branch with WMTS support

[jmwenda]

This has GeoExplorer WMTS support and Leaflet WMTS support to be used in the layer detail page and registering WMTS endpoints as external services within GeoNode.

[simod]

@jude this is very nice. Reading over the PR I see two separate works, one is the client side support for WMTS both on leaflet and on the worldmap geoexplorer and another is the services support for WMTS.
What we are trying to keep in this branch is to develop worldmap features on top of a vanilla geonode so that we can follow its developments. For this reason imagine this PR split in two, one against geonode master with the services work and one here with only modifications in the contrib/worldmap app.
The leaflet work it maybe go in geonode master if there are no plans to use it in worldmap.
I will take care of pulling from master once the PR gets approved.

[jmwenda]

@simod thanks was not really sure about adding WMTS support to vanilla GeoNode without the accompanying client side scripts. Will issue pull request to GeoNode

[jj0hns0n]

@jmwenda can we kill this one. I think it got merged upstream?