F50: Optional Fields

This merge request implements feature requested by the client.

Artwork database model:

• A lot fields in the database are now optional
• Longitude & latitude values are truncated automatically
• The field number was renamed to the field identifier and allows for text input
• The field linked_artwork was added
• The field campus and the corresponding database model were added

ArtworkMedia database model:

• Indices for ordering of media files are generated automatically

Campus database model:

• Added

Media database model:

• The field copyright was added
• Removes old version of media file when updated

Furthermore:

• Added scripts for backup & updating
• Added a license
• Requires the environment variables STATIC_ROOT & MEDIA_ROOT to be set