Make sure local URLs in image description are always absolute

File::getDescriptionText() parses the description page internally
for local files, but uses a HTTP request to action=render for
remote files; the parser nehaves differently in these two cases.
This patch uses a somewhat dirty trick to get rid of that
difference.
Bug: 57187
Change-Id: Ib7a6234bf79473fc333d2eef9f7d9ce6e6565141