Feature request: Allow logo in header to be changed

Currently it is not possible to change the logo in the header with out changing the source code of the frontend as the Mobilizon logo is hardcoded with SVG XML.

For a broader adaption some form of customizability is important IMHO. I understand that a complete theming setup is a lot of work (see: #751 (closed)). Having an easy option to change the instance logo (via the instance setting or overriding a file) could be good short term solution for having a more custom look of an instance.