Add lower case codec string for IMSC1 Text profile

[porcelijn]

Fixes #2378

• register TTML handler for stpp.ttml.im1t codec
• adjusted HLS test case to comply with rfc8216bis section 4.4.5.2
  This change keeps the legacy support for codec string stpp.TTML.im1t. Note that while unit tests pass without this, there is good reason to assume that there is still plenty content out there using the legacy content string.

To test:

• HLS: https://demo.unified-streaming.com/video/tears-of-steel/master_hls_fmp4_clear.m3u8
• DASH: https://demo.unified-streaming.com/video/tears-of-steel/tears-of-steel-ttml.ism/.mpd

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[porcelijn]

@googlebot Yup, I signed it

[googlebot]

CLAs look good, thanks!

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[ismena]

@porcelijn LGTM, let me run your PR through our build bot

[shaka-bot]

All tests passed!

[ismena]

Looking good, thanks for the PR!