46733 move filter dropdown from font awesome to our own icons ee

What does this MR do?

This MR updates the icons used in filtered search dropdowns and adds a utility function spriteIconPath that returns the asset path of Gitlab SVG's. The following icons are now used in place of Font Awesome:

• Search: search
• Author: pencil
• Assignee: user
• Milestone: clock
• Label: labels
• My reaction: thumb-up

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/46733

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46733