refs #8868
- Removed image-size in blog logo fn for meta data and made it synchronous
- Renamed `image-size-from-url.js` to `image-size.js` (incl. the test)
- Added second fn `getImageSizeFromFilePath` that reads from local file storage
- Added guard in `getImageSizeFromUrl` that checks if the image should be on local file storage and uses the new fn then instead
- Added a fn `fetchDimensionsFromBuffer` that takes the file buffer and returns an `imageObject` with dimensions.
- Added a new utils.js in `adapters/storage` for getting the file storage path
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
