Moved Core Theme Logos to CSS

In order to make it easier for a user/developer to replace logos in the core theme using a basic custom theme, I removed the hard coded `logo.svg` and `logo-wide.svg` from the core theme html, and replaced them with `div`s that use css `background-image` instead.

This has multiple advantages:
* The user can now use any filename they want for the logo.  They are no longer forced to use `logo.svg` or `logo-wide.svg`.
* The user can now easily use a JPG, PNG or GIF logo instead of SVG (SVG's are awesome, but used by an unfortunately small percentage of web developers at the current time.  Most probably don't even know how to make an SVG…).
* The user doesn't have to resort to hacking the core theme template files in order to use a different filename or file type for their logos.
* Greatly simplifies documentation instructions for how to change the logos (http://owncloud.org/theming/).

Signed-off-by: Jake Wilson <jake.e.wilson@gmail.com>