Add `ticket_id' field for tickets.

This field is in addition to the `id' field. We'll use this id for
ticket collection, since the actual `id' field provides an indicator
into the number of tickets sold.
Change Ticket model based on new `ticket_id' field.

The `ticket_id' field is required in the model. Thus, the Tickets
controller has also been changed, in the `create' method. The
`ticket_id' is generated by concatenating the user's id with the number
of tickets the user has. This is guaranteed to be unique. Then, it's
padded to the right with `0's, until it's 6 digits long. 6 digits are
sufficient, since there are only xxxx users, and each user can only have
x tickets. (The extra character was added to be safe.)
Change Facebook and Twitter logo size in footer.

Also changes the `vertical-align' property, which improves alignment
with the copyright text on the left hand side of the footer.