skins: replace {js,css}.template with actual files. Simplify startup scripts.

Currently, an Etherpad skin requires the existence of 6 files:
- index.{css,js}
- pad.{css,js}
- timeslider.{css,js}

In the default empty skin (in static/custom), there were 2 small placeholders
({js,css}.template) to be copied in place by the startup script in case no skin
was in use.

Now that we are moving to multiple directories (see #3471) we can simply commit
the example files and remove the copying code from the startup script.

src/static/custom/pad.css

@@ -0,0 +1,8 @@
+/*
+  custom css files are loaded after core css files. Simply use the same selector to override a style.
+  Example:
+      #editbar LI {border:1px solid #000;}
+    overrides
+      #editbar LI {border:1px solid #d5d5d5;}
+    from pad.css
+*/