cache the step validation results from Sympy

[bhpayne]

Every page that includes the derivation steps takes a long time to load because Sympy is validating the steps.

The validation only needs to occur for new steps or steps that have been edited. Caching the results would improve latency

[bhpayne]

Timing of two derivations with and without Sympy: https://physicsderivationgraph.blogspot.com/2020/05/timing-page-rendering-with-and-without.html

[bhpayne]

idea: hash the strings associated with each step (input, output, feed, inf rule) and store the hash + validation result per step in the SQL database.

[bhpayne]

ac1593857c8f6750037405a9feec365d638b5a0b