prefer logo png from theme over svg from core

The logo images are specified in the php templates with eg `image_path('', 'logo-wide.svg')`. If that file exists the correct path will be served to the client in the html template. Then the `SVGSupport()` is checked in https://github.com/owncloud/core/blob/master/core/js/js.js#L701 which will replace `svg` with `png` in img tags and css background definitions.

fixes #5074
partially solves #5421, but not when an svg has been specified in css and is then 'fixed' by js to point to a png which does not exist in the theme but is then might be resolved to an image from core .... well theoretical problem ... might not even be a problem

@karlitschek @jancborchardt please review