Find a way to increase memory limit for repos

[yuvipanda]

@a-paxton and I were talking about one of her binders which is undergoing peer review (https://osf.io/ur543/). We've determined that it actually needs 6G of RAM. I think its' reasonable to provide this, by limiting the total number of concurrent running users to something like 100/(6/2), so about 33. This means we are capping the 'total amount of resources being used by a repo', which seems reasonable.

Things that need to happen for this:

1. Allow configuring RAM per repo
2. Deploy a change that lets that happen.

[betatim]

I think being able to specify a regex/filter as part of the binderhub configuration that gives more resources to repos that match the filter would be a good way to go. Don't think we should let individual repos decide for themselves, there is no real downside to the individual user saying "my repo needs all the RAM".

[alimanfoo]

Hi folks, I'm creating some executable docs for a genomic data release and having trouble getting anything meaningful to run on mybinder in less than 2G. Appreciate this may not be easy to achieve but in case it helps to have an additional user saying "we need more RAM" then I'm here :-)