Add a form to search needs.

This is currently separate from the organisation filter, such that they
won't both work at the same time. We should take a look at merging these
together.
Remove 'commit' field name from org filter button.

Rails adds this by default, but it's just cluttering up our URL
parameters.
Filter out empty strings from query parameters.

The need API should handle these properly, but it's best not to pass
them through anyway.
Line up the org filter and search controls.
Refactor filter/search CSS to reduce duplication.
Ensure search term is present in field after searching
Merge pull request #38 from alphagov/search-needs

Search needs