[#921] Add more docs about resource proxy
[#921] Correct a couple of typos in the docs
[#921] Extend resource proxy docs
Give an example which shows what exactly will not work. Explain how it gets around the same origin policy.
Merge branch 'master' into 921-resourceproxy-docs

Conflicts:
	doc/data-viewer.rst
Clean up merge
[#921] Edit the data viewer docs

Move the references to the resource_proxy extension in the text_preview
and pdf_preview sections into seealso boxes.

In the text_preview and pdf_preview docs themselves, don't say anything
about local vs remote files.

Change "can not" to cannot

In sections about individual extensions use singular eg. "this extension
is part of CKAN" not "these extensions are".

Change the reference of get_proxified_resource_url() to a :func:
reference, this doesn't make any difference currently but if we ever
added an API doc for the function to our Sphinx docs (e.g. via autodoc)
it would be hyperlinked to it.
Merge branch 'master' of github.com:okfn/ckan into 921-resourceproxy-docs
Merge branch '921-resourceproxy-docs'