fix($compile): allow data: image URIs in img[src]

Ref: 1adf29af13890d61286840177607edd552a9df97

BREAKING CHANGE: img[src] URLs are now sanitized via a separate
    whitelist regex instead of sharing the whitelist regex with a[href].
    With this change, img[src] URLs may also be data: URI's matching
    mime types image/*.  mailto: URLs are disallowed (and do not make
    sense for img[src] but were allowed under the a[href] whitelist used
    before.)