Bump ImageModal XBlock to v0.3.0
Bump ImageModal XBlock to v0.3.0

This adds a new, backward-compatible feature:
- customizable alt text for screenreaders

Previously, the alt text was always set to the display name.
While this is still the default, instructors can now edit the value
manually.

Stanford-Online/edx-platform,jbassen/edx-platform,jbassen/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,jbassen/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,jbassen/edx-platform,caesar2164/edx-platform