Add a warning and drop support for issues related to the reload command

Re-open of https://github.com/SpigotMC/Spigot/pull/138 After it was accidentally closed due to branch restoration (great timing on my end.)

OP:
The reload command is fundamentally broken. It is known to cause several issues such as crashes, memory leaks and in some plugins unexpected behavior or disabling. This is known by most large server owners but some smaller less experienced owners don't understand why these weird occurrences occur, and don't connect them back to reloading.

~~As a result, I propose disabling the command until it actually functions properly, in a similar way to how we disabled the timings command since there was an alternative that actually worked. The alternative case here being doing an individual plugin reload or restarting the server. 8?~~

~~As of last commit, this PR now moves the reload command onto the server side (instead of API with most commands) and adds a toggle in your spigot.yml, defaulted to disabling the command.~~

Now this PR is just adds a warning stating the reload command is not supported.