[#86] Add extra image validation

[krissy]

_Addresses backend component of #86_

What this does

• [x] Add extra image validation to restrict upload images to the following formats: .jpg, .gif, .png, .tiff, .png
• [x] Add clarification of acceptable formats in validation message
• [x] Add some tests for image validation
• [x] Converts all image uploads to JPEGs going forward to simplify things and avoid TIFF issues

  Screenshots

Validation on upload attempt of unsupported format:

Green placeholder preview when tiff uploaded (not yet converted to jpg):

[steverabino]

:+1: :+1: :+1:

[CathMollie]

@krissy Just noting tiff upload wasn't very successful in usability test - took a long time and displayed as broken link on 'manage drawings' page.

[steverabino]

Good save @CathMollie, Kris is going to do some conversion magic!

[krissy]

@CathMollie good to know! Tanya just tested and you're right. Steve found this could be an existing issue with the file management tool we're using so working on a fix as part of this branch.

[krissy]

Tiff issue fixed in 11e5bfc - please review. Now all images will be converted to jpgs going forward

[steverabino]

FYI @krissy for tiff, the preview isn't working; I guess the image hasn't been converted by this point.

Is there a way to do the conversion before showing the preview; or shall we not worry about this for now?

[krissy]

TIFF preview addressed in 39aedc3 and screenshot added to PR description. Browsers don't support TIFFs without a hacky js library, and images aren't saved (and thus converted to jpegs) until the drawing is saved. So for TIFFs we'll show an alternate green placeholder to indicate change.

On a side note, we need a separate ticket to add some client-side validation for supported image formats. E.g. if PDF upload attempted, we show a red cross placeholder and appropriate validation message reminding user about supported formats.

[steverabino]

:+1: Nice work - sorry I found the bug :(