Allow full filtering syntax in the UI URL and in the API

[xavierhardy]

The filters are quite powerful but can only be used directly from the UI. It is impossible to link a search result using a URL or to use those filters in the API (except when matching on state).

http://flower.readthedocs.io/en/latest/tasks_filter.html#task-filter-syntax http://flower.readthedocs.io/en/latest/api.html#get--api-tasks

[xavierhardy]

Fixed by #642 too.