Use AudioContextOptions.sampleRate when possible

[kpreid]

Upcoming API feature: an AudioContext can be created with a specified sample rate. https://www.chromestatus.com/feature/5136778254090240 https://webaudio.github.io/web-audio-api/#dom-audiocontextoptions-samplerate

This could be used to replace the explicit resampling done in ShinySDR's audio stream client and probably would do a better job.