Multisite: Add missing "contribute.php" file to User Dashboard.
This changeset adds a wp-admin/user/contribute.php file to allow the "Get Involved" link to work on User Admin.
This changeset prevents an unintended 404 that occurs when a logged-in user clicks the "Get Involved" link (located in the menu-bar) from inside of the multisite-specific User Dashboard.
Related: r56220, r56309, #23348.
Props ignatiusjeroe, audrasjb, tobiasbg.
Fixes #61122.