Themers shouldn't need to write config schema

Problem/Motivation

We have made Drupal 8 in many ways a lot better experience for themers and then completely thrown them under the bus by forcing them to write a config schema. Config schema is weird even to module developers much less themers.

Proposed resolution

1. Create a settings.yml with the defaults inside in one of the core themes. I'd recommend stark.
2. Tell themers to copy this and add their own settings.
3. Provide a script that derives a schema from the settings yml (reusing the theme_settings type as necessary).

Remaining tasks

Agree, code, commit, party.

User interface changes

None.

API changes

None.