message: Allow minus symbol at beginning of CAPTCHA answer
diff: <mat-label translate>LABEL_RESULT</mat-label>
  <input id="captchaControl" [formControl]="captchaControl" matInput type="text"
  placeholder="{{ 'MANDATORY_CAPTCHA' | translate}}"
  - aria-label="Field for the result of the CAPTCHA code" pattern="[\d]*">
  + aria-label="Field for the result of the CAPTCHA code" pattern="-?[\d]*">
  <mat-error *ngIf="captchaControl.invalid && captchaControl.errors.required" translate>MANDATORY_CAPTCHA
  </mat-error>
  <mat-error *ngIf="captchaControl.invalid && captchaControl.errors.pattern" translate>INVALID_CAPTCHA
  