Adds special case for download limit so it can accept zero or blank

Signed-off-by: Paul de Wouters <paul@hmn.md>
Allow blank value for global setting and 0 for unlimited

Signed-off-by: Paul de Wouters <paul@hmn.md>
Unneeded statement

Signed-off-by: Paul de Wouters <paul@hmn.md>
Merge pull request #2086 from humanmade/issue/2085

Issue/2085