feat(assignee-dropdown): Swap out old assignee dropdown trigger for <AssigneeBadge/>

[MichaelSun48]

This PR swaps the current assignee dropdown trigger out for a new component, <AssigneeBadge/> under the feature flag,organizations:issue-stream-new-assignee-dropdown-trigger. This feature flag is set to release only in LA currently (although if you're reading this, I have not merged my PR to options automator just yet. I will do that before merging this.)

Before:

After:

Notice how the height of the issue priority dropdown trigger also increases a bit to match the height of the assignee badge. If the organization does not have the feature enabled, this height change will not be applied.

This PR will also remove the appearance of the suggested assignees in the issue stream, aka this state:

If an issue does have suggested assignees, this new trigger will now just show up as Unassigned, but the suggested assignees and reasons will still be surfaced in the assignee dropdown menu.