Revert "Prevent Firefox from attempting to install the XPI"

This reverts commit 1d39a08282ef5385affc431810372bf66cdfa2ee.

This fixes serving files from extension actions. I didn’t understand
what I was doing back then; the redirect() is needed because it is a
special URL handled in pootle/urls.py to serve the files with
django.views.static.serve, so using normal HttpResponse() meant I was
sending the browser a text string not the actual file!

The issue of Firefox trying to install the XPI file instead of
downloading it remains unsolved.
Prevent Firefox from attempting to install the XPI

Instead of redirecting to the '/export/' URL, use a regular
HttpResponse() to serve the contents of the file with proper
Content-Disposition set.
Fix leftovers of tp.overview

It was renamed to pootle-tp-overview in
945069d6a5fa08b8b3d9d144ce4c34289d355d92.