Validate image URL file size and/or dimensions

[jaaneh]

A limit of 200kb should suffice for the file size. This can (probably) be checked via the Content-Length header. There's also libraries that could do this. Like this or this for examples.

We may also benefit of limiting the width and height of the image.

[Fermain]

A very generous proposal:

• Max width 1600px
• Max height 1600px
• Max weight 256Kb

We can use the same limits if/when we add uploads.