Cache system output info instead of storing in a DB

[lyuyangh]

Currently, we persist system_info in the database so we don't need to recompute it for every analysis. This, however, means that every time the system_info schema is changed, we run into backward compatibility issues. We agreed that we should instead treat system_info as something we cache. It will appear to the user that new output is generated every time they click on analysis.

Related issues that can be solved as part of this one:
#355
#360
#320

• Move fields out of System.system_info (system_info is defined by the SDK so explainaboard_web plans to move away from it. We should only store fields we actually use in the DB and should not care about the specific structure of system_info.)
• Implement a cache for system_info (regenerate system_info if outdated)
• Move cached data to cloud storage