Use lower bitrate for voice recordings

At present we open an Opus encoder without specifying a bitrate, which uses the default of 64Kb/s. This is probably a bit overkill for our purposes. Conversations specifies a bitrate of 32Kb/s for its recordings, which seems like a reasonable value.

This should be a relatively simple matter of setting the bitrate property to 32000 in the GStreamer encoder. It looks like we already do property settings in two places in gtk/voice_message_recorder.py: once on line 113 with a set_property call and once as part of file merging on line 448. I think adding the extra property in these two places should be what is needed to accomplish this.