As a project maintainer I would like to upload multiple source files via the UI.

Notes:
- It is assumed we will find a suitable js library to allow the user to select multiple files to be uploaded with a single post. Possible candidates include http://www.dropzonejs.com/ and http://blueimp.github.io/jQuery-File-Upload/
- This story does not cover zip file upload.
- The server must reject unsupported files, indicating when it does so.
- There should be an admin configured setting for the maximum number of files allowed per upload.
- There will be manual testing involved for this story.
- Decision to be made whether to stream files or use temporary files. The latter may allow for parallel uploads.