JSON: Simplify container tags + minor fixes

[samuelfangjw]

1. Rename ref names for consistency (standardise all _ to be -)
2. Clean up metaphrase tag. Currently, langle and rangle are added on the frontend. This PR moves the addition of langle and rangle to the sicp repo instead.
3. Simplify parsing of "container tags", defined as elements that have a title and children elements. Rationale: these are all parsed in the same manner on the frontend. This PR changes the JSON tag of all elements with similar behaviour to SECTION to the SECTION tag (e.g. SUBSECTION now tagged as SECTION in JSON files).

[martin-henz]

Great.