Competitions screen: Include ability for user to JOIN/UNJOIN EVENT on the Listing

[LAlexMHllc]

When user has not joined the event, the button text should be "JOIN EVENT". When button clicked: add the user to the event; display the user joined message; change the button text to "UNJOIN EVENT".
When the user clicks the button "UNJOIN EVENT": the user should be removed from the event; the message should be removed; change the button text to "JOIN EVENT".
Screenshot Example