Add UI for managing configuration updates

Problem/Motivation

Currently there is no UI. An appropriate one might be roughly parallel to the "Synchronize" form for staging changes from one environment to another (admin/config/development/configuration):

• Divide list into sections, one for each type of extension (module [probably including install profile], theme).
• In a table, list (in alphabetical order) extensions of the given type that have available updates. For each, offer a link to show all changes. This link would lead to the report provided by Configuration Update Manager for the given extension.
• Below the list, have a "Safe changes only" checkbox, checked by default. Unchecking it brings up a warning and a red or otherwise warning-like button to apply unsafe changes.
• If "Safe changes only" is checked, bOffer a form that lists all extensions that have available updates. utton says Apply changes. Click to apply all safe changes.

Proposed resolution

Remaining tasks

User interface changes

API changes