Replace SRT ingestion with WebVTT ingestion

[waldoj]

The Video class and the scraper assume that the provided format will be SRT. Modify that code to instead work on WebVTT files, now that Granicus has moved to WebVTT captions (#21).

Within class.Video.php, these methods must be modified:

• parse_sbv: clone, change to be parse_webvtt
• normalize_line_endings
• time_shift_srt: make this build atop parse_sbv, so it doesn't touch file contents
• eliminate_duplicates: eliminate this entirely, semi-ironically
• srt_to_database: make this build atop parse_sbv, so it doesn't touch file contents
• srt_to_transcript: eliminate entirely

[waldoj]

All of this needs to be tested out.

[waldoj]

Works great.