Add filters for representatives

[njoyard]

This PR adds a filter form that uses django-filters on the representative list with the following criteria:

• search: identical to the previously existing search box, searches representatives by name
• chamber: static drop down list to filter by chamber
• country: static drop down list to filter by country
• group: autocompleted drop down to filter by all kinds of groups (except chamber and country) : party, committee, delegation, ...

The group URLs (like /legislature/representatives/committee/European Parliament/Committee on Defense whatever) are now 301-redirected to an URL with querystring parameters in order to pre-fill the filter form.

TODO:

• [x] fix select2 style to match the rest of the form
• [x] ensure the filter form always submits to the root representative list url instead of the current page url (not really needed but just in case)
• [x] add a checkbox to choose active/inactive meps postponed until we have an easy way to turn options/selects into a nice bootstrap button group
• [x] add something more obvious when there are no results
• [x] add a media block to the root template to allow page templates to add to it not needed, we have a head block already
• [x] change the way groups are displayed in the ac (add abbreviation, make group type more obvious) doesn't seem possible currently with select2
• [x] add filter tests
• [x] check duplicate meps issue again
• [x] fix missing querystring in page links

Preview:

[codecov-io]

Current coverage is 90.30%

Merging #92 into master will increase coverage by 0.58%

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