New API for downloading LDML files from projects that allow sharing WS data

[rmunn]

Currently this adds a "ldmlzip" command to our hg command runner.

Will return 403 Forbidden if project does not allow sharing ws data with SLDR. Will also return same 403 Forbidden error code if project does not exist, to avoid possibly leaking project codes.

If project exists and allows data sharing, command will return a zipfile containing CachedSettings/WritingSystems/*.ldml from the tip revision.

Future plans: C# code will unzip the *.ldml files into a directory by project ID, then zip up the whole thing for the API result. Then we'll create a job to delete the LDML directory four hours later, to clean up.

Ideally we'd be able to start streaming the output zip file as we receive data from each project, rather than waiting until all projects have been accessed and only then starting to stream the output zip. But we'll see.

[rmunn]

Unfortunately it looks as though we won't be able to use ZipArchive to add files to the output stream asynchronously as they come in: we'd probably run into this issue where ZipArchive tries to write the end-of-zipfile directory synchronously when it's disposed, but Kestrel doesn't allow synchronous writes to its HTTP output stream. So we might have to extract everything, then send the zip file separately. Which means that the HTTP connection might time out, so it's possible we'll have to make two API endpoints, one to prepare the zip file and one to download it. Testing required.