message: API: Add checkEditableBy User.ts convenience method
  checkEditableBy is basically just a reverse checkEditUser. This could
  be useful, since the "currentUser"s in the API functions (routes) are
  plain "IUser"s (and thus one can't call checkEditUser directly).
diff: @@ -17,6 +17,7 @@ interface IUserModel extends IUser, mongoose.Document {
  isValidPassword: (candidatePassword: string) => Promise<boolean>;
  checkPrivileges: () => IProperties;
  checkEditUser: (targetUser: IUser) => IProperties;
  + checkEditableBy: (currentUser: IUser) => IProperties;
  forSafe: () => IUserSubSafe;
  forTeacher: () => IUserSubTeacher;
  forUser: (otherUser: IUser) => IUserSubSafe | IUserSubTeacher | IUser;
  @@ -192,6 +193,10 @@ userSchema.methods.checkEditUser = function (targetUser: IUser): IProperties {
  return User.checkEditUser(this, targetUser);
  };
  +userSchema.methods.checkEditableBy = function (currentUser: IUser): IProperties {
  + return User.checkEditUser(currentUser, this);
  +};
  +
  userSchema.methods.forSafe = function (): IUserSubSafe {
  return User.forSafe(this);
  };
  