Add link where users can enter a join code and join a team

[StoneT2000]

Uses this API route: https://github.com/acmucsd/acm-ai-api/blob/master/src/routes/v1/teams/index.ts#L95

On the all teams page, in addition to team creation form you can also copy paste a join code and click join and attempt to join a team. Should be a simple input element / form and should use the /:competitionName/add-to-team/ route to join a team with the join code.

This should also do error handling. E.g. if joining is unsuccessful, list out reason (e.g. server error or wrong join code).