ResourceLoaderImage: Use hashes for versioning instead of timestamps

ResourceLoaderImage: Use hashes for versioning instead of timestamps

Remove use of the deprecated getModifiedTime() method and incorporate
file information in getDefinitionSummary() directly, instead of relying
on the parent class to include getModifiedTime().

Change getDefinitionSummary() to append to the summary instead of
setting arbitrary keys in the array directly to avoid conflicts.
This matches the pattern used elsewhere.

Change ResourceLoaderImage to use file hashes instead of timestamps
to avoid needless cache invalidation. At Wikimedia these modules
tend to roll over every week due to git not storing timestamps.
See also T104950, 9112c9347b, f37cee996.

Change-Id: I5d019bfb991c3b8042d1db14a853ba46cc690315