Add support for custom user profile fields for some settings

Adding the appropriate custom profile fields (as checkboxes) allows users
to enable Decaf settings for themselves that have not been enabled at the
site level.  Custom profile fields should be named decafSkipSettingsBlock,
decafSkipNavigationBlock, decafFullAdminTree and decafPersistentEdit.
You can use none, all or any combination of these to allow users to have
individual control over whichever of these settings you wish.