Improve reactions on messages in the greetings channel

[al-muammar]

The current logic is that if anyone wrote anything to greetings channel, we remove DESCRIBE_YOURSELF role from him and add FULL_MEMBER_ROLE. We need to improve this logic by:

• Not reacting on messages from users that have already had FULL_MEMBER_ROLE
• Reacting only on the first message sent by user with DESCRIBE_YOURSELF role
• Writing an error message in case a user haven't set his/her gender