Get and store end date input for multi day trips.

[eshika]

This CL enables the user to enter an end date and stores that end date as part of the Trip entity in Datastore.

Note: This CL currently does not incorporate checks for valid input (that the endDate falls after the startDate, etc.). I have discussed the implementation of this with Chris and plan to incorporate this in a future CL. I also didn't include any styling for the endDate box because I'm waiting for the UnifySiteDesign branch to be merged.

[zpChris]

LGTM, but leaving two notes:

1. You should change all instances of tripDayOfTravel to tripStartDate to indicate that the variable does not represent the sole travel date anymore.
2. Right now, adding the "Trip End Date" input field makes the "Next" button go onto the following line. Eventually, we'll want to prevent this from happening. In the UnifySiteDesign branch, the width of the site is wider and should allow this input form without pushing "Next" to a new line, but we should verify that this is indeed the case when the CLs are merged.