Improve reservation length editing

This does two things:

1. defaults the reservation length to 1 hour when the length can be selected.
2. the saved reservation length is selected by default when editing a reservation (instead of defaulting to the 1st option)

I did this assuming that the select is used mostly for room reservations, which are usually one hour reservations (in my experience).

Also, my ruby-fu is lacking, so there might be a better way to do this?

Status changed to merged

mentioned in commit 72ae150b