consistently call settings settings, not "configuration"

Created by: sqs

Site configuration != settings, but our API and docs are confusing here. This updates (in a backward-compatible way) our API and docs to call things by the right name.

addresses #859

type Mutation
  {configuration => settings}Mutation
input {Configuration => Settings}MutationGroupInput
type {Configuration => Settings}Mutation
  edit{Configuration => Settings}
  overwrite{Configuration => Settings}
type {Configuration => Settings}Edit
type Update{Configuration => Settings}Payload
type Query
  {configuration => settings}Subject
  viewer{Configuration => Settings}
type User
  {configuration => settings}Cascade
type Org
  {configuration => settings}Cascade
type Site
  {configuration => settings}Cascade
interface {Configuration => Settings}Subject
  {configuration => settings}Cascade
type {Configuration => Settings}Cascade

Test plan:

1. Manually confirmed that src-cli does not use any deleted fields
2. Tested with the released version of the browser extension with "Use extensions" both on and off, as both an anonymous viewer and as an authenticated user. Tested that an extension that calls updateConfiguration works on GitHub.
3. Tested an extension that calls updateConfiguration on Sourcegraph.
4. Tested saving user, org, and global settings.