Update Service Desk to use updated Vue standards (and remove from main bundle)

What does this MR do?

Update Service Desk to use updated Vue standards

• Remove Vue from main bundle, https://gitlab.com/gitlab-org/gitlab-ee/issues/2383
• Use .vue files
• Fix bug where we always show an error when the checkbox is enabled. Introduced in https://gitlab.com/gitlab-org/gitlab-ee/commit/f387a164210913ac417bf04e1ea12d6d058fb118

Are there points in the code the reviewer needs to double check?

• If we aren't comfortable adding the refactor into 9.2, I can create a slimmer version that just adds in the new bundle.

Why was this MR needed?

• Vue is included in the main bundle
• Previously when the checkbox got enabled, we always showed a fetch error
• Previously, Service Desk Vue wasn't using the new Vue standards (.vue files, styleguide)

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2383 (closed)