schema_filter view: allow multiple values for each parameter

[slinkp]

It would be really nice if you could select more than one value for each param on the schema_filter view. For example, in the boston demo, for restaurant inspections, you can currently only search for one kind of violation at a time. Why not more than one?

(Note the Columbia Tribune has specifically asked for this; currently working on it for them)

Looking at Chosen javascript library for UI for this, which would be a lot easier to do if #266 was fixed. Want to do that anyway.

[Paul Winkler <slinkp@…>]

(In [7303fdb4037265cfad71e363e3812d694ad85c13]) More work on lookups UI, refs #267:

• Factor out common code from schema_detail() and schema_filter()
• Use the chosen UI in the filter_lookup_list.html page so we can select multiple there too
• Use plural name of schemafield in schema_filter UI, to make it more obvious you can pick many

[Paul Winkler <slinkp@…>]

(In [6bb6c984f0e2640c33f65ed7a56d966de289afb9]) schema_filter: All inputs now use forms rather than links. Using Chosen js widget for selecting Lookups. Refs #267.
Still to do: Merge the display of existing values with the forms.
Details:

• bump up max number of lookups to show in UI, since we have a nice Chosen widget
• also change name of max lookups from LOOKUP_MIN_DISPLAYED to LOOKUP_MAX_DISPLAYED
• show count of available lookups if there's more than LOOKUP_MAX_DISPLAYED
• change some CSS selectors so as not to clobber Chosen styles
• more comments on filter_left_nav.html

[Paul Winkler <slinkp@…>]

(In [680c60629e0e94bf7abcf5dfb2eec65e38bde5fc]) Progress toward using select widgets for filtering by schemafield; refs #267

[Paul Winkler <slinkp@…>]

(In [1d084e244bc00621fd99a26f091e867e8c591a59]) Template tag for generating hidden form inputs to submit to the schema_filter view. Refs #267
Also refactored the filter_url template tag code to share code w/ the new tag.

[fkh]

Doesn't seem to be working for me --

• Go to  http://columbia.openblock.org/police/
• Choose the first two filter items (traffic stop, 911 calls)
• Click Go
• Results incorrectly include other items -- e.g. "C&I; DRIVING", "HARRASSMENT" --  http://columbia.openblock.org/police/filter/?by-incident_type=87%2C3

[Paul Winkler <slinkp@…>]

(In [55d367d9ede768e74aa49c32fbe80809cbe6e2d2]) Fix error in NewsItem?.objects.by_attribute() with m2m lookups; looking for [3,47] was finding any number starting with 3 or ending with 47. Joy of regexes. Refs #267