Fixes #1996 - Jail should also block gamemode changes

[caojohnny]

PR Synopsis

Fixes #1996

This PR introduces a change that would prevent jailed players from having their gamemode changed.

Rationale

As stated in the original issue, "Some plugins like CreativeParkour attempt to teleport and alter the player's gamemode. The teleport is blocked of course, but they end up in creative gamemode."

The changes proposed cancel PlayerGameModeChangeEvent if the target player is currently jailed in order to resolve this issue.

Impact

This is a very broad method of blocking players from changing gamemodes. If an admin wanted to change the player's gamemode, they will need to unjail them before doing so and then rejail them as necessary.

People who might expect the gamemode of jailed users to be capable of change (such as the fictional admins mentioned above) may find this to be inconvenient. However, I personally don't know of anyone who would need to do this. Again, it is still possible to change the player's gamemode by unjailing and rejailing in the potential rare case that it is necessary to do so.

[pop4959]

Code looks fine, I'd suggest avoiding the * import though

[caojohnny]

@pop4959 Is there any particular reason why that should be the case? Is the goal here minimal diffs or is there some other benefit that I'm currently unaware of?

[pop4959]

Mostly just nitpicking, which is why I approved anyways. Feel free to google the pros/cons of using * imports. Normally, there isn't really any noticeable difference though, so it's a fairly trivial matter. IntelliJ automatically merges imports into a * import when you hit 5, unless you change the default settings within the IDE.