Onboarding experiment popovers

What does this MR do?

Show popovers to guide users that are part of this experiment to the "GitLab onboarding" issues board.

The popover should act according to the following rules:

• If a cookie named onboarding_issues_settings does not exist, don’t do anything
• If the current requested step setting from the cookie is set to false, don't show the popover
• If the current requested step setting from the cookie is set to true, show the popover
• When showing a popover, set all previous steps in the cookie to false
• When taking the action suggested by the final popover, remove the onboarding_issues_settings cookie
• When dismissing any popover, remove the onboarding_issues_settings cookie

To try out this feature, run the following commands in the console to enable the experiment:

Feature.enable_percentage_of_time(:signup_flow_experiment_percentage, 100)
Feature.enable_percentage_of_time(:onboarding_issues_experiment_percentage, 100)

Then signup for a new account, go through the create group and create project steps and navigate manually to the just created group dashboard page.

Screenshots

Popover 1	Popover 2	Popover 3
design	design	design

There is no changelog, since all changes from this MR are behind the experiment onboarding_issues, which is controlled by the feature flag onboarding_issues_experiment_percentage.

Parent issue: gitlab-org/growth/product#107 (closed)

closes gitlab-org/growth/engineering#5364 (closed)