Fixes filtering on multiple activities
Fixes filtering on multiple activities

Previously, if a researcher selected multiple activities in the filter
page (with the expectation of getting an OR filter on runnables), the
filter would return zero students.

This was because the request was filtering

`... IN ('RunnableType_Id1, RunnableType_Id2')`

instead of 

`... IN ('RunnableType_Id1', 'RunnableType_Id2')`

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