Upgraded from v9 default styles to v10 wherever the developer expects to get the latest and greatest, as well as in a couple tests where it may be beneficial to ensure that we can handle a two-digit version number in the style URL.
This adds a `convertJSON` template, to be used like:
Error error
optional<Foo> foo = convertJSON<Foo>(string, error);
Internally, it parses the string with RapidJSON and then calls `convert<Foo>(parsed, error)`.
While here, rationalize GeoJSON converters and fix error handling for Tileset conversion in OfflineDownload.
