* initial cut at backed mode
* make flask multithreading conditional on debug flag
* update X access to support backed mode
* lint
* improve help message for backed mode
* fix tests
* add MatrixProxy to normalize supported matrix types
* add FAQ entry for --backed
* remove use of matrix.T
* clean up
* add ability to disable diffexp from CLI; add hueristic to detect likely slow diffexp calculation, and warn user
* fix tests
* do not print diffexp speed warning if diffexp is disabled
* tweak wording of diffexp speed messages
* add FAQ entry on --disable-diffexp
* revise heuristic for warning about slow diffexp
* use quick tooltip delay on diffexp button
