Search a route associated to an outing: add activity in the simple search filters

[desnoes]

cf. UI issue 2004

• modify \views\search.py to retrieve the selected activities
• modify \search\search.js to add the activity. This concerns the following functions: search_for_types and do_multi_search_for_types. The modifications should be propagated to all usages of these 2 functions.

Additional modifications are likely required.