Implement FilterService.getDisplayValueMap()

[jniles]

We're recently undertaken a large refactor of our filters to ensure that human readable ids are presented to the user when they filter the registries instead of large uuids or irrelevant numbers. One of the last PRs to land will be #2206.

Each search modal controller now contains the following code to create a map of id:displayValue:

var lastViewFilters = SOME_SERVICE.filters.formatView().customFilters;
var lastDisplayValues = lastViewFilters.reduce(function (object, filter) {
    object[filter._key] = filter.displayValue;
    return object;
}, {});

This code could easily be included in the FilterService on the client. Instead, of the above 5+ lines of code, a search modal should just do this:

var lastDisplayValues = SOME_SERVICE.filters.getDisplayValueMap();

to the same effect. It would reduce the complexity in our search modals and the total lines of code overall.