Give proof tabs different symbols

Per review comment by @XanderVertegaal in #79.

Author: jgonggrijp

frontend/src/app/annotate/annotation-menu/annotation-menu.component.html

@@ -16,7 +16,7 @@
     </li>
     <li [ngbNavItem]="2">
         <button ngbNavLink>
-            <fa-icon [icon]="faTree" class="me-2" aria-hidden="true" />
+            <b class="me-2" aria-hidden>⊤</b>
             <span i18n>Proof / Entailment</span>
         </button>
         <ng-template ngbNavContent>
@@ -29,7 +29,7 @@
     </li>
     <li [ngbNavItem]="3">
         <button ngbNavLink>
-            <fa-icon [icon]="faTree" class="me-2" aria-hidden="true" />
+            <b class="me-2" aria-hidden>⊥</b>
             <span i18n>Proof / Contradiction</span>
         </button>
         <ng-template ngbNavContent>

frontend/src/app/annotate/annotation-menu/annotation-menu.component.ts

@@ -4,7 +4,6 @@ import { FontAwesomeModule } from "@fortawesome/angular-fontawesome";
 import { NgbNavModule } from "@ng-bootstrap/ng-bootstrap";
 import {
     faSquarePollHorizontal,
-    faTree,
     faPenNib,
 } from "@fortawesome/free-solid-svg-icons";
 
@@ -34,6 +33,5 @@ export class AnnotationMenuComponent {
     public active = 1;
 
     public faSquarePollHorizontal = faSquarePollHorizontal;
-    public faTree = faTree;
     public faPenNib = faPenNib;
 }