VxDesign: Validate image uploads

[jonahkagan]

For seals:

• Background is transparent (if not SVG) --> Can punt on this one
• Minimum size: 200x200px
• Maximum size: TBD - investigate this as part of this task

Reject the upload and return an error message if any of the conditions isn't met.

Also consider:

• Clerk signatures
• Ballot measure images

Generally safe to assume: 1 seal, 2 clerk signatures, 1 ballot measure image

Clerk signature notably doesn't go into the election package.

Note that non-seal images are converted to base64 SVGs, so they grow in size. Current limits on upload end (5MB) apply to original upload, not converted file. Let's check the converted file.

Main guiding principle: Don't crash VxDesign or slow down VxSuite with gigantic election defs. Ballot looks okay.