feat: list dashboard templates from repository

[pauldambra]

Problem

splitting out from #12753

We want to list dashboard templates from an external repository. Installation and management will be similar but a chunk simpler than plugins

Changes

• Adds a DashboardTemplate model
• the model has a github url that is used to load the template
  • in follow-up this will be used to determine if updates are available
• Adds the OG template as a hard-coded always available template
• Uses that template when creating the OG dashboard
• lists templates in a tab in the dashboards section
• shows if they are installed
• shows if they are official
• allows installation of templates
• can use the installed templates as dashboard creation source

with feature flag off

with feature flag on

for follow-up

• check for and allow upgrades
• add instructions on how to create templates in the github repository
• add the templates requested in https://github.com/PostHog/posthog/issues/12732

How did you test this code?

added back end developer tests used locally with and without flag on

[posthog-bot]

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