[#1697] Don't break PEP8 tests if test files have lines >79 chars

According to naming test methods [1] in our testing coding standards, we should
make the test names clear "...even if it means your method name gets really
long, since we don’t write code that calls our test methods there’s no
advantage to having short test method names".  Given this, we shouldn't
validate E501 on our test suite.

[1] https://ckan.readthedocs.org/en/latest/contributing/testing.html#naming-test-methods
[#1697] Revert blacklisted files removed in cc7e917

They still fail even without validating E501.
[#1697] Unblacklist test files that are passing PEP8
[#1697] Fix minor PEP8 issues
[#1697] Add comment explaining why we're ignoring E501 on tests
[#1503] Clarify data vs metadata licenses

Fixes #1503
Merge pull request #1728 from ckan/1503-two-licenses-second-try

[#1503] Clarify data vs metadata licenses
[#1725] Remove useless code

This was moved to the Datastore plugin, so there's no need to be here anymore.
Merge branch '1697-dont-validate-e501-on-tests'
[#1725] Add test to guarantee that the filters received are in a dict
[#1697] Simplify _is_test() a bit
[#1697] Remove ckanext/resourceproxy/tests/test_proxy.py from blacklist

It's passing now.
[#1725] Add tests asserting that the IDataStore.where() works for delete

It should be used both on datastore_create and datastore_delete.
[#1725] Add delete_data() extension point and remove where()

@amercader and I agreed that, even though in most cases both select_data() and
delete_data() would create the WHERE clauses with the same code, it's easier to
understand the WHERE being created inside each one of those, not on a separate
where() extension point.
[#1725] Rename IDataStore -> IDatastore

This makes it consistent with the plugin name (DatastorePlugin).
[#1725] Reorder arguments on search_data() and delete_data()

This keeps our current 3 extensions points, search_data(), delete_data(), and
validate_query(), with the same initial 3 arguments, in the same order. The
interface becomes a bit more consistent.
[#1697] Fix PEP8's E303 on ckanext/resourceproxy/tests/test_proxy.py
Merge branch 'master' into 1725-extend-datastore
Merge branch '1697-dont-validate-e501-on-tests'
Merge branch 'master' into 1725-extend-datastore