Altered WCS link generation method width/height

Fixes #1356; WCS links should now return their native grid height and width
based on the WCS describeCoverage spatial grid.
Also changed how links are generated so that when fetching URLs on
very coverages the server does not need to issue several getCoverage for the
 entire grid extent of the coverage, which would ordinarily bog down the server
for large coverages.
Merge pull request #1373 from benjwadams/1356-fix_wcs_link_raster_resolution

Altered WCS link generation method width/height
Add Tweet button Under share this layer
Created a new template social_links.html and incuded it in layer_detail, map_detail and document_detail
Added a template at /avatar/add/ which gives correct message when a avatar of size >1Mb is uploaded
Added a check whether user has uploaded a file or not before clicking on uploading and also improved the errors page to redirect them to the error div in documents_upload.html
Merge pull request #1398 from mishravikas/fix-1395-1396

Error handling while uploading a document
Fixed download link at document_list.html so that now it points to the correct download link
Merge pull request #1397 from mishravikas/fix-1390

Fixed download link at document_list.html
Merge pull request #1394 from mishravikas/bug1357

Added a template at /avatar/add/
Removed tweet button which used javascript, added new tweet button which does not use javascript
Merge pull request #1393 from mishravikas/fix_1391

Fix 1391 Added Tweet Button under "Share this Map" and "Share this layer"
Removing the import to geomodels search (using geodjango) for now, as it is breaking search.
Refuse from previous commit.
Reverting last commits, as it is breaking things (sorry!).