message: update url to point to browser
diff: export const BLOCKSTACK_HANDLER = 'blockstack'
  export const BLOCKSTACK_STORAGE_LABEL = 'blockstack'
  -export const DEFAULT_BLOCKSTACK_HOST = 'https://blockstack.org/auth'
  +export const DEFAULT_BLOCKSTACK_HOST = 'https://browser.blockstack.org/auth'
  export const DEFAULT_SCOPE = ['store_write']
  export const BLOCKSTACK_APP_PRIVATE_KEY_LABEL = 'blockstack-transit-private-key'
  export const BLOCKSTACK_DEFAULT_GAIA_HUB_URL = 'https://hub.blockstack.org'
  