Move filter dropdown from Font Awesome to our own icons
We currently use Font Awesome icons in the main filter dropdown. We should use our own icons:
- Search:
icn/search
- Author:
icn/pencil
- Assignee:
icn/user
- Milestone:
icn/clock
- Label:
icn/labels
- My reaction:
icn/thumb-up
- Target branch:
icn/arrow-right
cc. @victorwu