Set request in template context

[aminalaee]

Closes #482 according to this.

Sets up request for template context automatically. So instead of this:

async def homepage(request):
    return templates.TemplateResponse('index.html', {'request': request})

We can do:

async def homepage(request):
    return templates.TemplateResponse('index.html')

So the context argument to templates.TemplateResponse is not required anymore.

[lovelydinosaur]

Okay, I've been looking through this for review.
I hadn't appreciated a couple of things until just now...

• I think this change will end up setting Content-Length: 0 on template responses. (Which would be broken.)
• Switching TemplateResponse so that it renders at the point it is called, rather than on __init__ is a bit of a behaviour changing thing for use to do. If response.body is inspected directly in the endpoint, it'll be b"".

I'm not sure what the upshot of those ought to be for us. One option is that we just decide this wasn't the right thing for us to do, and drop those issues. Another would be to make sure we set the Content-Length header properly, and then just be okay with the other behaviour changes.

?

[aminalaee]

I have to admit I didn't know about the first issue. I thought we just had to live with the second one.

I think unless we find a clean way to achieve the desired outcome, if possible avoid request in context but return proper response and have stable behaviour for Response, we should drop this and leave it as is.

Update: I'll spend more time on it and see if that's possible.

[lovelydinosaur]

I guess let's just drop this for now, and close the related issues as "we've decided not to do this". (?)
It'd be very nice if it was less awkward. But it's not, so.