Denying project membership requests

# Problem

Currently, when a user clicks "Join Project" on the project page, it sends a membership request to the owner/admin of the project. The owner/admin of the project can approve or deny the request. If the owner/admin denies the project, the user who requested to join won't receive a message telling them so. This could lead to a few situations.

1. Nothing.

2. The user asking to join will assume that the request didn't go through, so they request again, adding noise to the project owner/admin's request feed. The project owner/admin could deny again and the same process could repeat. Or, the project admin/owner will simply keep the request in their inbox, not accepting, not denying which would give the user a "membership pending" under their profile picture every time they visited that project.

# Possible Solution

When the project owner selects "deny", they will be asked to give a reason before they are allowed to actually submit the denial. The reasons could be populated and selected in a drop down menu. Potential reasons could be: Too many contributors with similar skills, not enough work for skill set, other, not taking on new coders, etc.

Adding the extra step could also decrease accidental denying, where the project owner/admin meant to click approve, but instead hits deny.