Add "Ticked by default" option for recurring contributions

We've had requests for the addition of a checkbox in the Contribution Page settings that will allow the "I wish to make a recurring contribution" checkbox to be ticked by default when the contribution page is loaded.

This seems like a fairly trivial modification and I've worked up a PR that I'm happy to submit for review. The PR adds an option to tick the recurring contributions checkbox by default (and wires that up to the page/form generation).

As this is my first commit, more than happy for honest feedback.