Add center and station filters for filtering final results by center and station.

[JohnMwashuma]

Task check list

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• Add a filtering section on the form results template for filtering results by center and station. Inside the filtering section create the following components:
  • A multiple select drop-down list of centers and stations.
  • An Apply button with some guidance text that says Filter results by center(s) and station(s).
• Create an on-change event listener for the centers select list similar to this that will be used to auto select the stations id's that belong to the selected centers. The on-change listener makes an Ajax request to this get_centers_stations function that retrieves a center's station id's.
• Create an on-click event listener for the Apply button similar to this that will be used to send an Ajax request to this django view for filtering the results by the selected center and station id. Ignore the else block in this implementation.
• Update the Django view filter_queryset method that filters results, to also filter results by selected center and station when available.

Related to

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#357