Make GitLab Faster: DeCruft Main.js

main.js is loaded at 587kb which has tons of crap in it that could be code split. cc @mikegreiling

@jschatz1 This will help make that file slimmer: https://gitlab.com/gitlab-org/gitlab-ce/issues/37220

marked this issue as related to #37220

mentioned in issue #38017

mentioned in issue #37952

changed milestone to %10.2

added Discussion label

We need to first remove everything from global namespace and add in code splitting. Also let's break this into multiple issues. @filipa @mikegreiling can you help here?

changed milestone to %Backlog

Roadmap is here https://gitlab.com/gitlab-org/gitlab-ce/issues/38869

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14734