`videosequence` and `problemset` have been replaced with `sequential`.
`problemset` and `videosequence` are old-but-not-entirely-unused aliases to the `sequential` block type (in Studio-speak, "Subsection").
Since [these block types have been removed from the 6 courses that used them](https://openedx.atlassian.net/browse/DEPR-151?focusedCommentId=588197), this ticket removes the support for the `problemset` and `videosequence` block-types.
For more information, see ticket: [DEPR-151](https://openedx.atlassian.net/browse/DEPR-151)
-- Changed docstrings to use double quote.
-- More consistent naming for youtube IDs (normal_speed_id -> youtube_id_1_0).
-- Formatting changes for readability.
-- Updated test XML to expect JSON-encoded strings; prevents tests from spuriously failing.
