Email verification code (#1382)

* Base verification code implementation, must define command, manager, events
* VerificationManager, verification command and messages, handled some sensible commands, configuration values
* Improved manager and sensible commands trigger
* Updated messages
* Updated verification code manager, fixed tests
* Switched to a permission based command
* Verification manager and command improved and added tests
* Edited messages

src/main/resources/verification_code_email.html

@@ -0,0 +1,20 @@
+<h1>
+    Dear <playername />,
+</h1>
+
+<p>
+    This is your temporary verification code for the server <servername />:
+</p>
+<p>
+    <generatedcode />
+</p>
+
+<p>
+    This code will be valid for the next <minutesvalid /> mins!<br />
+    Use the command
+    /verification <generatedcode />
+    to complete the verification process.
+</p>
+<p>
+    See you on <servername />!
+</p>