Allow passing parameters to preload

When pages are loaded in the edit box via preload, allow parameter
substitution. The interface-style $1 is used rather than the
template-style {{{1}}} to avoid conflicts with preloads that add template
parameters. Syntax is:
action=edit&preload=Foo&preloadparams[]=first&preloadparams[]=second

Bug: 12853
Change-Id: If02cf4b3dba9f9d22a956d8bfff224677cbce00d