fix(cslapps): update App.image_url

[krishnans2006]

Proposed changes

• Use CharField instead of URLField
• Lower max_length

Brief description of rationale

• The validators used by URLField don't allow for updating the field in Django Admin, when in the form /static/img/cslapps/app.ico.
• The current max_length is unnecessarily large (and could cause errors with some database backends).

[coveralls]

coverage: 80.142%. remained the same
when pulling 9e373ef on krishnans2006:dev
into 21911f1 on tjcsl:dev.

[NotFish232]

LGTM