Add a multiple select filter to the Experiment Area filters in proposal view

[georgeflecknell]

The Experiment Area filter needs to be multiple select to allow more than one instrument's proposals to be returned at any one time.

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BAU development time. From Instrument Scientist request, approved by Emma Gozzard.

[georgeflecknell]

This refers specifically to selecting multiple instruments in the FILTER, not selecting proposals with multiple instruments.