Add support for bulk download of user provided image file

[brindakv]

User provided image files are used on the PDB-Dev website.

We need a mechanism to download these files in an automated manner for release process.

mmCIF and JSON files and validation reports are downloaded using BDBag for entries set for release. Adding images to the BDBag will be a good option. The image files will need to be renamed as entry_Accession_Code.png or entry_Accession_Code.jpg.

Another option would be to create a command line tool.

The user provided image file can be renames to entry_Accession_Code.<suffix> when Submission_Complete--> HOLD is triggered, which is when accession codes are issued.