Ignore BOM at beginning of JSON
According to JSON rfc:
8.1. Character Encoding
JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default encoding is UTF-8, and JSON texts that are encoded in UTF-8 are interoperable in the sense that they will be read successfully by the maximum number of implementations; there are many implementations that cannot successfully read texts in other encodings (such as UTF-16 and UTF-32).
Implementations MUST NOT add a byte order mark to the beginning of a JSON text. In the interests of interoperability, implementations that parse JSON texts MAY ignore the presence of a byte order mark rather than treating it as an error.
At the moment json-glib is strict and fails for json files with a BOM (e.g. 0xFEFF). It would be good to respect the last sentence of the rfc section and handle it gracefully.
This would help supporting WebExtensions in Epiphany as those tend to distribute their translation jsons with a BOM.