Use an SVG icon for the show password button

As mentioned in commit 925f04e3f, SVG icons offer many advantages over
icon fonts.

app/assets/stylesheets/management/account/edit_password_manually.scss

@@ -5,6 +5,7 @@
   }
 
   .show-password {
+    @include has-fa-icon(eye, regular);
     cursor: pointer;
   }
 }

app/components/management/account/edit_password_manually_component.html.erb

@@ -11,7 +11,6 @@
       <%= f.password_field :password, class: "input-group-field", label: false, value: nil %>
       <span class="input-group-label">
         <button type="button" class="show-password" aria-pressed="false">
-          <span class="icon-eye"></span>
           <span class="show-for-sr"><%= t("management.account.edit.password.show") %></span>
         </button>
       </span>