[JIRAALIGN-1194] Roadmaps: Filter -> Epics does not respect T1 config

Issue Summary

When filtering the Roadmap by Epic, the filter does not respect the T1 config. In instances with a huge number of Epics, this can lead to performance issues and/or background 500 errors due to SQL timeouts

Steps to Reproduce

1. Find an instance with a lot of Epics. We are talking 2k+
2. Select a relatively light T1 config i.e. maybe only a few Epcis in the PI
3. Navigate to Roadmaps
4. Click Filter, Epics and . Do not select any other filter
5. Click into the selection box

Expected Results

The Epics pulled into the selection box should only be from the current T1 config

Actual Results

If a Theme is not selected, then all Epics are loaded into the available selection, resulting in performance issues for larger data sets. There is a note on the epic filter instructing the user that this is a sub-filter of theme, however, from a customer point of view it would be clearer if the filter loaded in line with T1

Workaround

Ensure that a Theme filter is also selected to decrease the number of Epics pulled into the available selection.