[ckan-dev] A couple of bug fixes to cherry-pick into 1.8

[Sean Hammond]

> I'd like these two to get into 1.8 if possible, needed for CMAP:
> 
> Fix for relative url being used when doing file upload to local storage
> cfdeb919f228f917b287eda203e5a5371e0977bc
> 
> group controller: use extras_as_string for group read
> a8eedda82c0c4e6a4fea5888e5b7864a4abef39e

and here's three more bug fixes for 1.8 which are needed to CMAP
(although if they didn't get into 1.8, CMAP could move to the 1.8.1
branch):

[#2750] Fix group controller using wrong schema
6a48063b971a5f6813c5034eace85e479b5308df

Refactor DefaultGroupForm's form_to_db_schema_options()
3ed510b2da164e566fc2c23eee84a495f6800cb3

[#2570] Fix IGroupForm check_data_dict not being called
5c63f301ae22f6f54a62afa0d0211be3cee628cb

Note that these affect code paths that are only there to be used by
extensions and are not used in core, and there are no tests in core that
touch these code paths. I'm working on example extensions with tests and
updated docs for this stuff in a branch
(2750-add-docs-and-examples-for-idatasetform-and-igroupform) but I don't
know when that'll be ready, so I wanted to get these bug fixes in now.

If you want to test manually, you can use example_igroupform on that
branch to test these bug fixes.