Use config instead of globals for ImageGallery

Use config instead of globals for ImageGallery

Have to pass a context to the constructor to acutally use it for
settings.

Also adds a RequestContext::getMainAndWarn to get a default warning,
when using the main request, but it would be better to pass one.

Change-Id: I1628a1790c45d44aa4239701486b8b1b7c59a0e6