Add image model with upload function

[BeritJanssen]

Requirements

• In Django, add a model with the following properties:
  • src (FileUpload field, I assume)
  • alt (text input)
  • href (url/link input)
  • rel (text input or multi select with the official rel values)
  • target (select with options null/None or _blank. we could add more options but probably not necessary. see also the target attribute docs)
  • tag (or tags, perhaps separated by commas/colons)

Context (goal behind goal)

This image model will (eventually) be used for several features in the nearby future such as:

• Experiment collection footer images Configure footer through admin: ManyToMany Image field, short description #826
• Theme images Configure ExperimentCollection logo through admin #822
• Experiment image Configure a description and image per experiment #831
• Visual Matching Pairs
• Dynamic OG image in Dynamically change page title, description and OG attributes based on current experiment #834