Webapp: Switched to consistently relative paths

As I have mentioned in [OpenHAB issue #853](https://github.com/openhab/openhab/issues/853), the WebApp UI is fairly inconsistent with its use of paths (some relative to the document root, others to the current directory). I have changed all URLs to be relative to the WebApp itself, which is more consistent and offers better compatibility with reverse proxies.
Also, the links to the CMD servlet were wrong (it resides in the same directory as the WebApp, not in its parent), it only worked due to the webapp directory already being the root directory and there being no "up" from root. Therefore, I removed the '../' in front of 'CMD' as well.