SlashCommandEducator should check length

[Zabuzard]

To reduce false positives, SlashCommandEducator should additionally check the length of the message and not send its advice if the message is longer (> 30).

Currently it only checks the regex:

So we should just add a simple && content.length() < MAX_COMMAND_LENGTH to that, with MAX_COMMAND_LENGTH = 30.

Also adjust the unit tests in SlashCommandEducatorTest, to add one of the real messages where it triggered falsely:

.isLetter and .isNumber are available