Fix downloading generated files

Passing the return value of ProjectIndexView unconditionally to
render_to_response() does not work anymore, so instead move it to
ProjectIndexView to be called conditionally as appropriate.

There are still some issue with the download, Firefox will try to
install the langpack instead of saving it and "Save Link As" will
suggest a misleading "Download langpack.htm" file name.
Prevent Firefox from attempting to install the XPI

By using HttpResponse() and "Content-Disposition: attachment", this
wouldn’t work with the redirect() call that was used before, but I’m not
sure the redirection is really needed after 1cf39da.
Control extension actions with waffle

Added an is_active() method that checks if the extension action specifies
a _waffle_flag property and returns if it is active or not.
Use lock file to prevent concurrent XPI builds

Attempting to run build_xpi concurrently can fail with non-obvious
errors, so it is petter to limit it to one build at time.
Separate building and downloading langack actions

So that we can allow everyone to download the packs, but only few to
build them.
Create extension’s flag if it does not exist

Defaults to disabling the flag for all, even superusers.
Fix Mozilla actions installation instructions
Use “Mozilla” group for all Mozilla actions
Use the same waffle flag for building langpack and downloading tarball
Show some feedback after building the langpack
“langpack” → “language pack” in visible strings
Limit Mozilla actions to firefox and mobile projects