Implement new metadata viewer

Remove the SimpleCSVPreview plugin, we're no longer using CKAN's preview
infrastructure because it uses an iframe, replace it with our own snippet
instead.

Add a new template snippet that renders the CSV table and the selected column's
metadata side-by-side, and lets you click on columns to change which column's
metadata is shown.

This is done without JS currently, so there's a different URL for showing each
column's metadata, URLs like /package/id/file/id/schema/index are used for
this.

The old View Schema pages are gone. The old Edit Schema pages are still there,
but they're now accessed by an Edit Schema button where the View Schema button
used to be, rather than by clicking on the columns in the CSV preview. This
schema editing should be replaced as well.

Tests for this still need to be written, may take some ideas from the old
SimpleCSVPreview tests.
Merge branch 'master' of github.com:ckan/ckanext-b2
Add dropdown menu for quick resource switching
Implement the rest of the resource page modifications from Sam's mockup

* Dropdown menu on the resource title for quickly switching to the pages
  of the other resources in the package.

* Added share button with dropdown menu of sharing links

* Removed the sidebar, where the package resources and sharing links
  used to be

* Other tweaks and simplifications of the page layou
Fix CSS on sharing links dropdown menu
Add headings to dropdown menus on resource page
Add some metadata viewer tests

More to come. Also added some error handling into the metadata viewer,
fixing a crash when viewing a resource that has no uploaded file.
Add some more metadata viewer tests
Don't crash when uploading a non-CSV file
Don't crash when previewing a non-CSV file
Merge branch 'master' of github.com:ckan/ckanext-b2 into date-type-autodetection

Conflicts:
	ckanext/datapackager/lib/csv_utils.py
	ckanext/datapackager/tests/lib/test_csv_util.py
Rename a couple of variables

Variableames buffer and type were overriding Python builtins
Delete an unnecessary # TODO comment