closes #953 add possibility to ovewrite graphiql html

[lioman]

If graphiql parameter of app is set to a str, we can use it to overwrite the built in template

[lioman]

closes #953

[lioman]

Not sure how to push this, or help with review. Would be nice to have this in starlette.

[erewok]

It seems like a potentially useful configurable parameter to add, but the type [bool, str] is one that I find confusing, especially in combination with the isinstance call later on: that variable is being coerced into a wide range of responsibilities. Consider how you would support, for instance, passing a str that represents possibly a template itself (such as the default in the code) or a filepath which could be retrieved and used as a template?

It may be helpful to look at other libraries as well. For instance, graphql-server has a config which for Flask offers the following options:

- graphiql: If True, may present GraphiQL when loaded directly from a browser (a useful tool for debugging and exploration).
- graphiql_version: The graphiql version to load. Defaults to "1.0.3".
- graphiql_template: Inject a Jinja template string to customize GraphiQL.
- graphiql_html_title: The graphiql title to display. Defaults to "GraphiQL".

[JayH5]

Thank you for creating this pull request. We have decided to deprecate GraphQL support within Starlette itself so I am going to close this pull request. See #619.