Add filter by number of assignees

[leodeleone]

Hi! I added a filter by number of assignees as you mentioned on issue #2. I think it helps people looking for unassigned issues :)

[iedr]

Hi, thanks for your contribution!

1.) The dropdown appears to have changed into a "dropup" (the menu opens upwards instead of the default downwards). You could refer to https://developer.snapappointments.com/bootstrap-select/examples/ under Dropup menu to check out how to rectify this.

2.) The options for the assignee filter are: 0 (731), 1 (227) etc. Maybe we can make it clearer by saying 0 assignee (731), 1 assignee (227) etc.