Add opening database to build mode

[GoodChessMind]

Although it's possible to have this using scid and chessbase it's really convenient to have this in studyopenings.com.

Option 1: Have a opening preset opening explorer already on studyopenings.com like chesstempo already does
Option2: allow user to import a database of games and view the opening moves as a tree

[jven]

Thanks for the feedback!

Note that importing multiple games as PGN is already supported! So if you can export those games as PGN from your database, you can already bring them into studyopenings.

But I agree there might still be value to also adding an opening explorer similar to what lichess has in its analysis board.

My hesitation to building this is that it's probably much easier to have users use ChessBase/lichess/Stockfish/etc running on the side while creating repertoires.

[jven]

More user feedback: "WOuld it be possible to have engine support while building the repertoire and a library of Grand Master games !"