Notebooks Idea: Allow for custom templates

we currently have our templates defined in `src/flows/templates/types`, and we are about to embark on a product experiment to see if our users find value in starting their notebooks from a template instead of just a metric selector and a visualization. If that proves useful through some sustained bump in notebook creation or resource generation, we should abstract the definition of those templates out of a folder in our codebase and setup some rules so that users can use their own templates from github.

it could be something as simple as adding a line under the "create from template" interface that shows a the string `https://github.com/influxdata/ui/master/src/flows/templates/types`, and allows them to edit it.

this'll allow organizations to set up their own prioritized list of templates that works best for their workflows, which we can measure to see if people want or not by looking at the number of orgs without default template links