- This fixes an issue caught by feature spec where
  the "Leave admin mode" needed data-method="post"
- It also fixes a handful of missing qa selectors

https://gitlab.com/gitlab-org/gitlab/-/issues/301143#note_561841810

- In a previous commit we iteratively introduced
  the structure.
- This commit iteratively fills in the structure
  with all the pieces missing pieces from the
  `_dashboard.html.haml` view.

- This provides a view model which will
  power both the responsive and wide view
  of the top nav.
- Introduces a PORO for TopNavMenuItem
- Adds some Builders to help improve
  readability and maintainability of this
- Migrates a fiew nav links from the views
  to here. These will be finished in a later
  commit.