linklist editor: add internal link support

+ The object ID that is returned by the mediabrowser is turned into a
relative URL in the form of “/#{id}” instead of an external URL. This is
save, because the mediabrowser only returns object IDs of CMS objects.
The API does not (yet) allow to get the relative permalink URL if a
permalink is set. It also omits the optional object name parameter at
the end of the relative URL. Possible GET parameters are also not
supported.

+ The linklist editor checks for external URLs to save them untouched.
Relative URLs are checked for a CMS object ID. If found, then an
internal link is created. The API does not (yet) allow to send a
relative permalink URL, so permalinks are not supported by the linklist
editor. If the object ID can not be found in a relative URL, then a CMS
alert pops up stating to either provide a URL or an object ID. Only
providing the ID (without a starting slash) is also possible, as it is a
valid relative URL.

fixes #278