Added media source support to plugin

[markwillis82]

trafficstars

Plugin now supports a media source (defaults to filesystem).

A drop-in extra that allows the gallery to use a media source (for example S3, or cloud files). I have updated the uploaders and the file paths in the get() function to allow for urls at the start of filenames.

We are currently using this in production and have no issues.

[jpdevries]

Gallery is now managed over at modxcms/Gallery. and am wondering if you could help me understand how to test it. Just not sure how to use the new feature. https://github.com/modxcms/Gallery/tree/media-sources