[#4818] add admin extensions
[#4818] add documentation for extensions
[#4818] don't error if tool doesn't exist any more
[#4818] sort entry points for reliable ordering
[#6482] import page for existing projects
[#6482] change page to show importers in a grid
[#6482] add a test for the importer admin page
[#6482] simpler logic to build matrix of importers (thanks Cory)