Make UX for misspelled or delted config much less bad

Summary:
Fixes T3436. Currently, when installs have configuration options which we don't know about, we raise a fairly confusing/ambiguous message about the options being unknown. Instead:

  - Keep a list of previously valid (but now deleted) config, with explanatory reasons for what happened to it. Present this information, along with altenate wording ("Obsolete Config" instead of "Unknown Config") where applicable.
  - Show a list of all the places the config is defined.
  - Provide an active link to delete it from the web UI.
  - Provide a command to delete it from the CLI.
  - Allow `bin/config delete` to delete configuration options which no longer have a definition.

Test Plan:
  - Set an auth key in database, local and file config.
  - Walked through the setup issue, cleaning it up.
  - Set an invalid key and made sure I still got a reasonable error (this now has better cleanup instructions).

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3436

Differential Revision: https://secure.phabricator.com/D6317