On a sub-wiki, we cannot use a Skin from another wiki or from filesystem anymore

Use Case:

• Reuse the same Skin to multiple subwikis;
• Have all the Skins in the main wiki and just call them from sub-wikis.

Currently we cannot select a Skin from the main wiki, since we have a select that lists the skins just from the current wiki.
Also in Edit mode we have the select.

The solution is to import the Skin in the needed wiki, but if you want to reuse in multiple subwikis, the code will be duplicatedin each imported theme.

Another use case which is broken is to indicate a filesystem skin.