Add 'Request Join' option to a Gitcoin Bounty Project

### User Story

As a user, I would like to be able to request to join a project directly through the project's page. Instead of the project members manually searching for my gitcoin handle and trying to add me to the project. There was an error for me in saving my teammate in the project as a contributor, and there was no alternative way for him to join the project on Gitcoin.

### Why Is this Needed

*Summary*: Currently, when a user "starts work" on a bounty, it means they need to manually add members via gitcoin handles. Although this is fine when you already have a team formed, it would be nice to have an option where anyone who reads up on our project would like to join in by sending a "request to join" through our project's page. Just allowing that option would result in a better experience in forming teams.