Disallow users who have not joined a trip from signing up for transportation

[Former user]

Users who are not signed up for a trip can request to ride in cars. This should be disallowed.

Current behavior

Users who have not joined a trip are able to click the "Ask to ride in this car" button on the transportation tab with no error message.

Expected behavior

One of the following:

1) Users who have not signed up for a trip should not be able to access transportation information 2) Users who have not joined a trip should not be able to see the "Ask to ride in this car" button 3) Users who have not joined a trip should be given an error message when clicking the "Ask to ride in this car" button

Steps to reproduce

1) Receive a link to a camping trip you have not yet joined 2) Click the transportation tab 3) Observe behavior stated above.

[hornd]

Welp, bitbucket formatting >:|

[Ke Zhu]

fixed on both carpool and lodging page