[#54] Support xsd:sequence elements when reading schema

This is in addition to the previous support whech was just xsd:choice
elements.

Note that the support for xsd:choice/@minOccurs has been removed, as it
is an incomplete implementation that is no longer needed.
[#55] Remove Nones from type column in schema table
[#25] Display element occur values in schema table
qualify codelist mappings with conditions

FIXME update the documentation of match_codelist, or refactor
add link to dashboard

FIXME should probably be a 1.04/1.05 change as well
Will get 404s for elements nobody is using, should the dashboard have
explantory pages for these?