improve purge command

[danthe1st]

This Pull Request

• removes @Nullable for null-checked variable
• Makes sure that /purge stops after not deleting any messages 20 times
• make sure not to delete more than amount messages (Previously, it always loaded amount (or 100 if amount is greater than that) messages from the channel even if some messages have already been deleted)
• load a bit more message when clearing messages from a particular user
• use channel#purgeMessages for bulk-deletion

Alternatives considered

• make the number of empty iterations to stop purging configurable
• make the amount of messages to load in case of deleting messages from particular users configurable