closes#8568
- use our `urlJoin` util to concatenate the URL (not the query part of it, as this is not supported in `urlJoin`) and to prevent possible missing or double slashes, as `config.apiUrl` could be with or without trailing slash
closes#8479
- removes `markdown` field from schema
- removes `legacyMarkdown` converter
- updates tests to work with `mobiledoc` field instead of `markdown` and adapt for mobiledoc HTML output where necessary
