Migration plan from Karma to Jest

Evaluating Jest as a new test environment was successful (https://gitlab.com/gitlab-org/gitlab-ce/issues/49171#note_107466861). This issue describes our plan for migrating the frontend test suite.

Phase 1

1. merge in minimal Jest setup (https://gitlab.com/gitlab-org/gitlab-ce/issues/53757)
2. add axios mock and fail for pending requests (https://gitlab.com/gitlab-org/gitlab-ce/issues/33715)
3. add Vue transform (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23686)
4. support EE-specific tests (https://gitlab.com/gitlab-org/gitlab-ee/issues/9081)
5. make Jest work with gitlab-ui (https://gitlab.com/gitlab-org/gitlab-ce/issues/56689)
6. convert a small amount of tests from Karma to Jest as examples

Phase 2

1. fail for Vue warnings and errors: https://gitlab.com/gitlab-org/gitlab-ce/issues/58203
2. support running Jest tests in headless Chrome: https://gitlab.com/gitlab-org/gitlab-ce/issues/58205
3. create Danger rule to favor Jest over Karma (as part of https://gitlab.com/gitlab-org/gitlab-ce/issues/49799?)
4. start porting existing tests by directory

Each of the steps happens in a separate merge request. After Phase 1 is completed everybody should be able to write tests in Jest.