Browser animation converter third-party notices

@ffmpeg/ffmpeg 0.12.15 is Copyright (c) 2019 Jerome Wu, MIT licensed.
The full MIT notice is LICENSE-MIT.txt in this directory.

@ffmpeg/core 0.12.10 is the unmodified single-thread FFmpeg WebAssembly build,
licensed GPL-2.0-or-later. See COPYING-GPLv2.txt. It includes FFmpeg n5.1.4,
x264, x265, libvpx, LAME, Ogg, Theora, Opus, Vorbis, zlib, libwebp, FreeType,
FriBidi, HarfBuzz, libass and zimg. Individual upstream copyright and licence
notices are included in each source archive. This software has NO WARRANTY.

Complete upstream sources and build scripts are available at the same location:
/static/vendor/ffmpeg/sources/manifest.json
The manifest lists downloadable .tar.gz archives with SHA256 hashes and upstream
source URLs. ffmpeg-wasm.tar.gz contains the source, bindings and Docker build
scripts, and Dockerfile-source.txt identifies the component versions and compiler
image (emscripten/emsdk:3.1.40). No codec or wrapper sources were changed here.

This distribution's package-lock.json is in browser-converter/ in the app source;
it records the npm archive integrity hashes. The deployment includes the source
archives so obtaining source does not depend on third-party hosting remaining up.