Convert CI/CD templates picker experiment into default state for empty pipelines page [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This cleans up the experiment code around the pipeline empty state templates feature as this experiment was considered successful. See here.

It also changes the functionality in the following way: instead of opening the single file editor when clicking a template, now the new pipeline editor is opened with the template.

A quick summary of the changes:

• The pipeline_empty_state_templates experiment code is removed
• What was previously the pipeline_empty_state_templates experiment, is now converted into the default experience
• What was previously the pipeline_empty_state_templates experiment, is now converted into the control state of the remaining 2 experiments
• The experiment tracking when a template is clicked is now converted into regular tracking
• The new pipeline editor now loads a template whenever a template URL parameter is present
• The mew pipeline editor is opened instead of the single file editor when clicking a template
• The Hello-World template is removed in favor of the 3 stage Getting-Started template

Issue: #326299 (closed)

Screenshots

Before	After