Edits to the sidebar text

Update URL terminology, minor edits, add note
Merge pull request #6187 from edx/carol/doc/DOC-1333

Edits to the sidebar text (DOC-1333/SOL-130)
Cleaned up some warnings
SOL-20 Fix test, remove usage of Text category because it was replaced
Added some mime types to include gifs and tifs within image filter