Refactor welcome controller code to reduce duplicating and increase clarity

Problem

A lot of duplicate and general refactoring can be done in the welcome controller to increase clarity and reduce the complexity.

Doing this will enable easier extension and maintaining of this area. It could also reduce the likelihood of future bugs that can appear due to complexity of code and paths.

original

The following discussion from !108278 (merged) should be addressed:

• @dstull started a discussion:

  There are a lot of items in this welcome controller that could use a refactor and cleaning up... spawning an issue

Plan

• Refactor welcome controller
  • !108575 (merged), !108706 (merged), !108278 (merged)
• Refactor/fine tune learn gitlab controller - !109162 (merged)
• Remove dead code paths in learn gitlab and onboarding
  • !109251 (merged)
  • !109170 (merged)
• Move learn gitlab index action to show
  • !111185 (merged)
• Move continuous onboarding to learn gitlab onboarding action
  • !109243 (merged)
• Move learn gitlab code to ee since it is only invoked there.
  • !109273 (merged)
  • need to move one remaining item in isOnLearnGitlab - !110578 (merged)