Inject breadcrumbs that point to project browsing pages

Splitting this off from #1803654: Missing the breadcrumbs on project pages so that can be closed...

In those patches, there are still some TODO comments about adding breadcrumb links to the project browsing pages (e.g. /project and /project/[project-type] in D6). The problem is that those pages don't yet exist, so there's no where to link to. ;)

At the very least, we need #1559946: Port project_solr to D7 to be completed.

However, it's kinda lame to depend on project_solr. D7 project isn't providing its own /project view, either, so we can't link to that, either.

I'm not really sure the cleanest way to make this conditional or alterable, either. An alter hook invoked from project_project_get_breadcrumbs() seems nasty. Maybe it's okay to just do an if (module_exists('project_solr')) in there for now.