Convert admin/settings controller specs to system specs

[mjankowski]

Related to ongoing discussion on #30195

This is sort of a minimal proof of concept of starting to migrate some of these, but also is viable to merge. Before I do a bigger set, let alone all/most of them, I want to make sure we're on the same page style-wise, wording-wise, etc - in regard to converting any of these.

For this one specifically:

• With the exception of the branding controller spec here, these were all very very minimal specs with barely any assertions - which is part of why I picked this as a starting point. All the conversions have the same exact coverage (hitting the show view, filling in a form, submitting form, asserting something about results) which is technically a coverage increase (none of the controller specs have any assertions about results beyond http response code). This could be further enhanced with some assertions about db/settings changes, but I wanted to keep it close to a 1:1 conversion for this first pass.
• This is a very slight LOC reduction from the controller to system specs (offset a bit by adding the helper in this PR), and preserves the 100% coverage that the controller classes had before.
• I anticipate having better abstractions and more helpers as we evolve this to more places (probably some helper gems as well for some of the capybara interactions?) but again, wanted to start simple here and not pre-abstract too much. I did a first pass with all of the specs very verbose/repetitive, and started to pull some stuff out, but then stopped myself before I went too far.
• There's a slight execution speed-up from creating fewer factories post-conversion, but also a small slowdown from exercising the full server/browser interaction ... more or less a wash there, and hard to measure given small size of things; but we may want to keep an eye on this as we convert more.