Add automated quality control checks before submitting data to Gemma

[arteymix]

As per https://github.com/PavlidisLab/GemmaCuration/issues/3#issuecomment-759691608, it would be valuable to perform some sanity checks before submitting data to Gemma.

Here's a list of basic things that we can do:

• detect samples with zeroed-out expression vectors (this could be caused by an alignment failure, or too short reads, etc.)
• ensure that every column have enough variance
• add a Luigi parameter to manually skip QC checks
• report QC checks in the Web viewer
• report QC checks on Slack (in case of failure)