Transaction sidebar, partials, and controller cleanup

[zachgoll]

Up to this point, we've done a lot of exploratory work with transaction-related views and have intentionally avoided any refactors to optimize for speed.

There are some issues that need to be addressed as they are starting to slow down development.

I'm keeping this issue somewhat general as there are likely improvements to be made that are not listed below in this initial roundup:

• Fix form submission issues with the transaction sidebar (it may help to pass a ?transaction= while on the /transactions view to know the currently opened drawer is and allow for non-turbo form submissions)
• Move controller logic to model
• Clean up model
• Consolidate and organize partials (there are a lot of similar names)

As mentioned above, there are likely more issues to address here, but the overall goal is to clean up the transactions module of the codebase a bit so we can continue to develop new features quickly.