Skin: Use WANObjectCache for sitenotice caching

Skin: Use WANObjectCache for sitenotice caching

* Move the md5() hash to the cache key, this makes it much safer
  after a change happens by avoiding write competitions between different
  servers and db slaves. It also allows an undo to re-use the existing
  cache if it still exists.

  In addition, it enables idiomatic use of getWithSetCallback given
  that get and set are now logically separated.

* Avoid fragile re-use of variable names. Previously it read the
  original $notice value at multiple points but also setting $notice
  to $parsed after a certain point. Consistently use $parsed only.

(Ref T115890.)

Change-Id: I5488cc894ff1544e6c20b7d51a7a2adfc292c4ec