Add trimming and tests to the sound editor

src/components/audio-trimmer/audio-trimmer.jsx

src/components/sound-editor/icon--trim.svg

src/components/sound-editor/sound-editor.css

src/components/sound-editor/sound-editor.jsx

src/containers/audio-trimmer.jsx

src/containers/sound-editor.jsx

test/__mocks__/audio-buffer-player.js

.../unit/components/__snapshots__/sound-editor.test.jsx.snap

test/unit/components/sound-editor.test.jsx

test/unit/containers/sound-editor.test.jsx