[migration] stop caching the mapping from constraint type name to eid It's not so frequent that a cache seems necessary, and we were not invalidating that cache when adding a new constraint type. Related to #3724892.

[web/json] an empty rset is just fine for ejsonexport The ejsonexport view can just return an empty list for an empty rset. Closes #4005518

[RichString] Add markdown support Supporting markdown requires the 'Markdown' python packages, and since it widely available on the target platforms adding it as a strong requirements will not be a big constraint. Closes #3814302