Added internal BitTorrent v2 support

[qstokkink]

Fixes #8457

Note: this is not the big refactor into libtorrent, just the bare minimum not to crash on v2 info dicts.

This PR:

• Adds v2 info dict support into the TorrentDef.
• Adds TorrentDef.get_file_indices() to fetch the file indices of the file list¹.
• Updates Files.tsx to use currentIndices instead of currentIndcices.
• Updates various GUI files to not depend on the file list length for indices¹.
• Updates the TorrentInfoEndpoint to no longer return the raw metainfo, but instead give the files and names directly (this was previously derived inside the GUI).

Footnotes

1. The list of files is not (or, no longer) equal to the list of indices (these include padding files now). ↩ ↩²

[qstokkink]

Oops, the commit is still named "WIP: Add internal BitTorrent v2 support".. renaming.