Allow multiple assignees to be chosen from dropdown filters

What does this MR do?

Enable multiple assignees in filtered search for issues page but not MR page

Are there points in the code the reviewer needs to double check?

Shouldn't be

Why was this MR needed?

Deliverable

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #1967 (closed)