Limit the size of SVGs when viewing them as blobs

What does this MR do?

This MR prevents rendering of SVG blobs larger than 2 MB.

Are there points in the code the reviewer needs to double check?

I couldn't figure out how to properly test the view for this as this requires a large SVG (inflating the repository size) or stubbing, the latter doesn't really seem possible using Spinach and all the stuff potentially involved.

Why was this MR needed?

When rendering large SVG blobs (e.g. 20 MB files) a request may time out and consume a lot of memory.

What are the relevant issue numbers?

#1435 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc @jschatz1 @annabeldunstone (there are some UI changes, see the above screenshot).