Icon size isn't enforced at all

[Anthony Lenton]

When loading a new app, the form validation doesn't complain if an icon of a different size is uploaded for each icon field.
The 16x16 field should enforce the uploaded file to be a 16x16px image (or an svg), and the same for the 32, 64 and 128px icons.

QA Steps:
1) Create a new app
2) On the Displaying your app form, try selecting an icon of a different size for the 64x64 icon
3) The form validation should respond appropriately: "The 64px icon must be exactly 64x64 pixels in size."
4) Repeat 2-3 for the other 3 icon fields
5) Select an svg for the 64x64 icon
6) The app should be updated correctly,
7) repeat 5-6 for the other 3 icon fields.

[Michael Nelson (michael.nelson)]

JFTR: we'd switched from softwarecenteragent.forms.ApplicationForm (pre-devportal) which did include the validation, to devportal.forms.ApplicationUpdatedForm which did not include the validation.