Add permission scope support to builder

[Blackbaud-PaulCrowder]

SPAs should be able to define a permission scope as a set of permissions to be retrieved when making authenticated calls via SkyAuthHttp.

[Blackbaud-PaulCrowder]

In blackbaud/skyux-builder#245

[Blackbaud-PaulCrowder]

Notes for documentation:

To specify a permission scope, use the SkyAuthHttp service in @blackbaud/skyux-builder/runtime and call its withScope() method, then chain the get()/post()/etc. to that method. Example:

import { SkyAuthHttp } from '@blackbaud/skyux-builder/runtime';

export class MyService
  constructor(private http: SkyAuthHttp) { }

  public getThings() {
    return this.http
      .withScope('my_permission_scope')
      .get(someUrl)
      .map((response)...
  }