Use a button to show/hide password in the management area

When using a link, people using screen readers might think they're going
to a new page where the password is going to be shown. With a button,
they get a better idea about what to expect.

Furthermore, with a button, we can use the `aria-pressed` attribute to
indicate whether the password is currently being shown.

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

@@ -10,10 +10,10 @@
     <div class="input-group">
       <%= f.password_field :password, class: "input-group-field", label: false, value: nil %>
       <span class="input-group-label">
-        <a href="#" class="show-password">
+        <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>
-        </a>
+        </button>
       </span>
     </div>