feat(users): add user invitation functions

[mjuniper]

AFFECTS PACKAGES:
@esri/arcgis-rest-common-types
@esri/arcgis-rest-users

[jgravois]

assuming that all these operations require auth, my recommendation would be:

export interface IInvitationRequestOptions extends IUserRequestOptions {
  invitationId: "string"; // mandatory param with more explicit name
}

// same for accept/decline
export function getUserInvitation (
  requestOptions: IInvitationRequestOptions
)
const url = `${portalUrl}/community/users/${username}/invitations/${requestOptions.invitationId}`;

see #78 for more info. i still really need to write some CONTRIBUTOR doc for this.

[coveralls]

Coverage remained the same at 100.0% when pulling 4c0fce1 on f/user-invitations into dd7a5ac on master.