Come up with a new example for extra_template_vars plugin

[simonw]

This example is obsolete, it's from a time before request.actor and authentication as a built-in concept (#699):

datasette/docs/plugins.rst

Lines 696 to 700 in 0c064c5

	@hookimpl
	def extra_template_vars(request):
	return {
	"auth": request.scope.get("auth")
	}

https://github.com/simonw/datasette/blob/0c064c5fe220b7b3d8dcf85b02b4e60452c47232/docs/plugins.rst#extra_template_varstemplate-database-table-view_name-request-datasette

[simonw]

I'm going to show how to display the current user's user-agent.

[simonw]

https://github.com/simonw/datasette/blob/c7d145e016522dd6ee229d4d0b3ba79a7a8877c1/docs/plugins.rst#extra_template_varstemplate-database-table-view_name-request-datasette