Add MT variant in substitutions panel

[ArchiDevil]

Users want to see machine translation variant in possible substitutions variant on right panel.

It would be done with a naive approach like "put a request to MT API every time substitutions are requested" but it leads to slow losing money from user's account every time they clicks on a segment. Some kind of caching is required when the file is processed automatically. If file is not processed with MT on uploading, then no MT substitutions are available.