feat: Dashboard templates in onboarding

[raquelmsmith]

To help people get started with PostHog product analytics faster, we want to enable them to use dashboard templates in the onboarding flow. This will:

• Get them to insights faster - exactly what templates are good at
• Help them leverage autocapture events - they are hard to figure out how to use, so people dont' see how powerful they are
• Help intro them to the toolbar on their website
• Intro them to Actions, which are a super cool part of PostHog

Rough mocks from Cory:

To dos:

• Add feature flag that this can be developed behind (feat: add flag and onboarding steps for dashboard templates #23069)
• Add dashboard template selector and configurator to onboarding steps (feat: add flag and onboarding steps for dashboard templates #23069)
• Add iframe to load customer's site, using hosts from ingested events (feat(onboarding-dashboard-templates): get the iframe loading #24692)
• Add option to "select from site" for template variables
• Link the toolbar action creator to the template feat(onboarding-templates): create actions from within the iframe #24825
• Update toolbar action selector UI to work for this
  • Let people say if they want to include the element text, href, and page url
  • Hide existing actions section
  • Change button text to just "Select element"
• Make sure people can only use this if they have haven't installed the snippet
• Dashboard creation confirmation screen
• Let someone use pageview as a variable
• Make sure this only shows up if there are actually any templates (eg self-hosted won't have any templates - but maybe it should?)
• Make the iframe loading timer thing work for this - pull apart from heatmaps
• Audit existing templates for:
  • Make sure the template makes sense to use here
  • Make sure variable names and descriptions make sense for this new flow
  • Add templates we need?
• Let people select "any pageview" or "any screenview"
• Make it clear people can edit these later
• Add analytics / tracking
  • Make sure we can differentiate between insights / dashboards made through this and through manual creation
• Let someone switch the site URL (eg view pricing on posthog.com, sign up on us.posthog.com)
• Fix toggle color in action popover
• Let someone edit the URL with regex
• Use separate keyed logic for the variables
• Add feedback form
• Add "I need a different template" option
• Skip on mobile? Or make mobile friendly.
• Hide the docs panel when someone gets to this screen
• Maybe just skip altogether if they don't have posthog-js events?
• Don't let the iframe be a s long as the variables, constrain to vh