* DEV: Add API to alter uploads Markdown
* DEV: Extract data attributes from image / download Markdown
For example '[test|attachment|hello=world]' will generate an 'a' element
with a data attribute: 'data-hello=world'.
This commit also makes MarkdownIt to transform '|attachment' into
'class="attachment"'. This transformation used to be a part of the
process which resolves short URLs (i.e. upload://).
* DEV: Export imageNameFromFileName
* FIX: move attachment_css_class constant out of upload-short-url for discourse-markdown-it
* Use setTimeout instead of ember later
* WIP. Not sure if this worked.
* oneboxer cache in separate file
* Reset onebox cache still
* set functions for oneboxers cache
