Fix multiple issues with notebooks completing execution

[chmreid]

There are several notebooks that encounter exceptions or fail to complete execution. The easy solution is to simply ignore these notebooks during treon tests, but we should go back through each of these notebooks, uncover the problematic code, and fix it so that our users can successfully run the notebooks as well.

Specific examples:

• in the "Login" notebook, there are several bash magic commands that (unnecessarily) try and actually run the hca login command, which fails, crashing the notebook
• in the "Find cell counts" notebook, the elasticsearch query returns 0 results, crashing the notebook
• several of the "download" notebooks (definitely 10x seq data nb) refer to non-existent resources or have queries that return 0 results