Add optional player argument to ::setlevel

What has been done in this MR?

• Made the ::setlevel command take an optional argument consisting of a player name for whom to set the level. This should make it easier to test things that require not being an admin and still having a certain level, like hax had recently needed.

What should testers check?

• "::setlevel skill level player" successfully modifies somebody else's level (should also work on yourself)
• "::setlevel skill level" successfully modifies your level
• that no exploits were introduced accidentally (I fail to see how I could've, but that's par for the course)

• I have tested these changes thoroughly.

• I used the relevant Thanos/Zaros tool for any JSON edits where possible, and have attached screenshots of any changes.