Solidus: Rename one letter variables

[spaghetticode]

Throughout Solidus' codebase there are many one-letter variables. These are not ideal as they are not expressive and meaningful for human beings.

I think these should be replaced with human intelligible words, as @jacobherrington already did with some in #3292.

This is a beginner-friendly issue, especially suited for anybody interested in starting helping with open source.

[mkoltonski]

Hi
I can handle it if you don't mind

[jacobherrington]

Hey @mkoltonski I would love to have your help with this. Here is a good trick to find these variables:

# in the solidus repository
gem install reek
reek --smell UncommunicativeVariableName

That should give you a list of them 👍

[kennyadsl]

Thanks to @mkoltonski, @JDutil and @juliannatetreault for the work on this issue!