Moving uploaded attachments out of the database. Working on #54.

* Added file upload service
* Added fileUrl attribute to Attachment (content still exists so we don't corrupt legacy data)
Adding database image migration service. Working on #54
Added migration action to AttachmentController. Working on #54.
Fixes #54. To run the migration just use the  endpoint.
Merge pull request #83 from OxBRCInformatics/feature/54_images_in_domain_class

Soheil says it's fine :). Peer reviewed with Grant and @SoheilSaif.