Enable shareable, UI-created experiment templates

Templates are "formulas" for experiments that can be used to populate experiment metadata, stages, mediators, and agents. Currently, experimenters can add a "template" to the template gallery by submitting a TypeScript file to our open-source code base and having us deploy it.

Moving forward, it'd be more efficient and flexible if template were created in the UI and did not require programmatic configuration.

Some example user journeys:

• Creation: A user creates an experiment. In addition to a "Save as experiment" button, there is also a "Save as template" button.
• Sharing: Templates can be private, sharable (same way as experiments), or global. As inspiration, think Doc templates in Google Docs.

Open questions:

• How do we want to handle updating of templates?

What would need to happen before this:

• Making sure there is UI parity for key features; are there features in the code templates (such as cohort survey moving based on ID) that do not exist in the UI?

Start by reviewing the existing code-based template gallery and comparing its features with the UI. Identify gaps such as cohort survey movement based on ID before defining the implementation. Done should mean users can create templates in the UI, save them, control whether they are private, sharable, or global, and understand how updates work.