Swap place of assignee and author selectors

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

Now the assignee dropdown selector is left of the author

I propose to have the author first and the assignee second

The reasons is that time flies from left to right and the author is determined before selecting an assignee.

/cc @dzaporozhets @JobV @skyruler @creamzy

Added frontend label

@sytses The reasons is a nice one. There is an amazing book by Susan Weinschenk "100 things..." with lots of reasoning behind intuitive design patterns.

Milestone changed to 8.6

Added ~131895 label

mentioned in merge request !2131 (closed)

mentioned in merge request !2256 (merged)

mentioned in commit 1be406d7

Status changed to closed by merge request !2256 (merged)

mentioned in commit 7945cf41