Support for enabling skins in the installer

Let the user choose the skins to install (like extensions)
and decide which one will be the default for the new wiki.

Right now core skins are always enabled regardless of user
choices there, but I'm working on that.

Bug: 66440
Change-Id: I2e10720a6ac327e66c415bb91dc897885f952738
Merge "Support for enabling skins in the installer"