Onboarding experiment apply novice filter

What does this MR do?

Hide advanced issues for novice users.

When users that are part of the "onboarding_issues" experiment that have selected the "novice" option when asked for their experience level visit the "GitLab onboarding" issues board, only show the issues with the "Novice" label applied to them.

Screenshots

To try out this feature, run the following commands in the console to enable the experiment (and if you have DNT enabled in your browser, please turn it off):

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, choose novice and navigate to the "Learn GitLab" project's issues board.

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

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.