Disable editing field layouts when allowAdminChanges is false

[stevecomrie]

Basically, since the entire site's Settings control panel section is hidden when allowAdminChanges is false it makes sense to hide this section as well to prevent front-end users from being confused by these options.

Additionally / alternatively, it might be nice to have some increased flexibility for setting individual user permissions to hide some other features on a production site that might not be editable.

For example, disabling the ability to Add new Ad Groups, or alternatively the ability to hide the "Ad Groups" menu item entirely if it doesn't make sense for them to be edited on production.