Broadcastify uploader fixes - 5.0

[taclane]

Three relatively minor edits for the broadcastify plugin in rc/5.0

1. The Broadcastify plugin currently re-reads the call json from disk as part of the initial upload stage. As of [#927], this disk read is no longer necessary and potentially undesirable now that the json can be directly dumped from from the Call_Data_t object into the curl object. The dumped json is also stripped of whitespace and should be smaller than the formatted file Trunk Recorder generates for human review.
2. The filename field does not appear to be in use with the current Broadcastify API. Additionally, call_info.converted provides an absolute filename for the audio, and such uploads may unintentionally transmit details about the host system and/or home directories/usernames that are not required. Broadcastify appears to ignore this field, and automatically generates filenames for audio as [SystemId]/[startTime]-[talkgroup].m4a using details provided during the upload.
3. The plugin does not presently use the log_header() formatting function to simplify logging statements. These have been added where useful.