Add /op and /deop commands

[satoshinm]

Adds two simple convenient commands, /op to add a player to Admin rank, and /deop to add to Default rank.

These commands just call out to /rank, which can already be used for this purpose, but /op and /deop are more intuitive. The lack of /op was confusing for myself when first starting to use Cuberite, and not only me: cuberite/cuberite#3684 cuberite/cuberite#1835 it is also in the FAQ:

https://forum.cuberite.org/thread-1596.html

Q: How can I make myself operator?
A: You can use the rank ...

but #158 notes a caveat, the Operator rank does not have full privileges, by "opping" a player the admin likely intends to add to the Admin rank instead. This pull request implements the suggestion:

Example: To get Operator in Cuberite you need to execute /rank [player] Admin(Note that the Operator rank has not the full permissions!), but most players are familar with /op [player]. So the /op command should be defined as a shortcut to the /rank ... Admin command.

[NiLSPACE]

This won't work if someone removed or renamed the admin rank.

[madmaxoft]

You should add a support function that would search through all rank for the one that has a * permission, using cRankManager:GetAllRanks and cRankManager:GetRankPermisions. Don't forget to handle the case when no such rank exists (fail the /op command with a proper message).

[satoshinm]

Changed to iterate to find the '*' permission rank for /op (GetAdminRank()), and use the default rank for /deop (GetDefaultRank()), removing the hardcoded Admin and Default rank names, hows this?

[madmaxoft]

Looks good, thanks.

[NiLSPACE]

Thanks, this will likely prevent many confused users :)