Replace VITE_RECIPIENT_JOIN_DEADLINE to automate it based on round end time

[yuetloo]

This issue was based on a review comment for PR: #694 to automate the hiding of the "Add Project" button based on a round's end time.

Review comment:
Add an environmental variable to automate this so that if there's less than this amount of time remaining in the round, then the UI will hide the "Add Project" button, and redirect the corresponding routes back to the "Projects" view (just in case someone tries to navigate to them directly.