Adding a new filter for <select multiple> (MultiSelectDropdownFilter)

[lrljoe]

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Adds a new filter type (MultiSelectDropdownFilter) which is a version of the MultiSelectFilter, along with updated tests and documentation

[lrljoe]

Note that this branch has now had additional updates to include a "setFirstOption('value')" to set a default (top of list) option, that when selected also removes the filter from the query.

[gpibarra]

should this be "MultiSelect Dropdown Filter"?