Hide follow request buttons after responding

Fixes #106

fedireads/templates/snippets/follow_request_buttons.html

@@ -1,3 +1,5 @@
+{% load fr_display %}
+{% if request.user|follow_request_exists:user %}
 <form action="/accept_follow_request/" method="POST">
     {% csrf_token %}
     <input type="hidden" name="user" value="{{ user.username }}">
@@ -8,3 +10,4 @@
     <input type="hidden" name="user" value="{{ user.username }}">
     <button type="submit" class="warning">Delete</button>
 </form>
+{% endif %}