Have meek-client-torbrowser write the native host manifest.

The WebExtension needs a JSON "host manifest" that both authorizes the
extension to run a native executable, and tells the browser where to
find the native executable. The path inside the manifest needs to be an
absolute path, so we cannot just plunk down a static file; we have to
know the path to where the browser is installed. meek-client-torbrowser
rewrites the manifest on each startup, where the browser expects to find
it.

The is mostly self-contained and compatible with previous behavior, with
one small exception on windows. On mac and linux, the browser expects to
find the manifest in a well-known location (relative to $HOME, which in
our case is inside the browser's directory tree or the ancillary
TorBrowser-Data directory). But on windows, the path to the manifest
needs to be stored in the registry. So meek-client-torbrowser not only
writes the manifest file, it also writes a registry key pointing to the
file. I'd like to try and find a way to do this that doesn't require
modifying global state like this.

This patch is tested on linux and windows but not mac.