Admin: Add context.mergeItem() to merge server values into forms

[lehni]

• Merge only the given values as if loaded: deserialized with the whole item, lists and objects merged by id, defaults and computed values run again
• Keep the user's edits of other values and the clean or dirty state
• Merge into drafts, and in place into the parent's item for forms with mutate, like resetItem()
• Use mergeItem() in the guide's polling example instead of Object.assign()