Incremental refactoring of Static Site Editor controller and config

What does this MR do?

This MR contains iterative refactoring and test improvements in preparation for adding the Static Site Editor config file as part of #241164 (closed).

This is Iteration 1 of a three-part iterative approach to implementing the Static Site Editor config file.

Config architecture refactoring:

The main change is introducind a decoupled/cohesive separation between the handling of configuration values which are generated versus those which will be read from the config file, as discussed and approved by the SSE team here:

Other refactoring and test improvements

• Move logic from views/controllers to library classes. ("skinny controllers", "passive views")
• Move behavior out of constructors into instance methods. For example, access to repository. This allows usage of preferred mocking approaches like instance_double and allow_next_instance_of
• Naming consistency - make names of same values consistent across classes, and prefer to match what they are called when passed to the client:
  • payload -> data
  • file_path -> path

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Review App Static Site Editor Link

https://gitlab-review-add-sse-co-a627sr.gitlab-review.app/gitlab-instance-administrators-dfcd66ee/gitlab-self-monitoring/-/sse/master%2FREADME.md

Related Issues/Epics

• #241164 (closed)
• &4267 (closed)