Add support for search queries in RSS feeds

[jorgecarleitao]

Currently the RSS feeds are defined by us. However, since we can provide filtered lists, we should also be able to provide them via RSS. The objective here is to extend our feeds to allow using parameters (GET), and filter the results accordingly.

One way to accomplish this (let's say for the contracts) is to build a new function out of the part of the contracts.views.build_contract_list_context that filters the query, and use it to build the feed and in build_contract_list_context. The same can be made for the others.