Add a mechanism to store custom links per-project

As part of the d.o redesign of the project node UI, if you look closely at the "Links" block, you'll notice an "Add new link" link (presumably just for project maintainers). There are all sorts of existing hard-coded links, but there's currently no way to have custom links. We're going to need some way to handle this part. Probably a new column to hold a serialized array in {project_projects} is all we need, and a UI to specify them, and some code in the forthcoming project_get_project_link_info() function (or whatever we end up calling that over at #372580: Move the project node links to blocks).