This adds the required tracking for the new paid signup flow.
We're using frontend tracking as well because it gives us a user
session. For users who have not activated frontend tracking, we
can fallback to backend tracking.

Two things to be aware of:

1. Users who go to /-/subscriptions/new who are not signed in, get
redirected to /users/sign_up. After they successfully created an
account or signed in, we redirect them to /-/subscriptions/new again.
However, when they go to -/subscriptions/new, we would track the
experiment_start event a second time. Therefore, we added the param
experiment_started to not call track them twice

2. For the sign_up_view event, we only want to track users who got
redirected from /-/subscriptions/new, therefore we have added the
redirected_from=checkout param.

Prevent default overwrite for theme and color ID in user API

See merge request gitlab-org/gitlab!26792

Refactor notebook_viewer to vue SFC

See merge request gitlab-org/gitlab!27090

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27090

Move approximate counters to batch counting

See merge request gitlab-org/gitlab!27218

Migrate ee/custom_metrics to Jest

Closes #194274

See merge request gitlab-org/gitlab!27139

Move Caching module out of GithubImporter namespace to be reused

See merge request gitlab-org/gitlab!27096

Move finders related to CI under the namespace

See merge request gitlab-org/gitlab!27190

Add package_type as parameter to group packages list api

See merge request gitlab-org/gitlab!26833

Added package_type param to api options
Updated tests
Updated documentation
Added changelog entry

Move feature category of ProjectUpdateRepositoryStorage to Gitaly

See merge request gitlab-org/gitlab!27165

Replace underscore/lodash in feature_flags

Closes #210264

See merge request gitlab-org/gitlab!27130

Refactor pdf_viewer to sfc

See merge request gitlab-org/gitlab!26894

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/26894

Add disqus redirects to moved docs

See merge request gitlab-org/gitlab!27221

Send email notifications for Generic Alerts docs

Closes #207488

See merge request gitlab-org/gitlab!26856

Enable MR :single_mr_diff_view feature flag by default

See merge request gitlab-org/gitlab!24821

Support DotEnv Variables through report type artifact

See merge request gitlab-org/gitlab!26247

Remove ~bug label from QA Failure template

See merge request gitlab-org/gitlab!26989

Turn on new_variables_ui feature flag

See merge request gitlab-org/gitlab!26901

Add `options` parameter to `convertObjectPropsToSnakeCase`

See merge request gitlab-org/gitlab!25569

This commit adds support for `artifacts:report:dotenv`

Minor text changes

Make the changelog title make
more sense to users.

Use timecop to lock date to 20200312 for certs

Closes #210557

See merge request gitlab-org/gitlab!27239

Performance development docs: Link to mass insert

See merge request gitlab-org/gitlab!27083

Optimize Project related count service desk enabled

See merge request gitlab-org/gitlab!27115

Cleanup what services are running on Gitaly standalone

See merge request gitlab-org/gitlab!26060

Fix capitalization on HAProxy

See merge request gitlab-org/gitlab!27131

Added JSON License

See merge request gitlab-org/gitlab!26122

Add docs for the edit custom metric link

See merge request gitlab-org/gitlab!26518

Add deprecation note for Severity and Confidence levels

See merge request gitlab-org/gitlab!26984