Switch +/- on comment collapse buttons with CSS

This was previously being done a bit manually in the JS and HTML, but
CSS can handle it like this, which simplifies some things (and also
takes the +/- out of the DOM, which is probably good in this case).

tildes/scss/modules/_btn.scss

@@ -46,6 +46,10 @@
   }
 }
 
+.btn-comment-collapse-label::after {
+  content: "−";
+}
+
 .btn-comment-label {
   @include min-touch-size;
 

tildes/scss/modules/_comment.scss

@@ -192,6 +192,10 @@
     font-size: 0.6rem;
     line-height: 0.8rem;
   }
+
+  .btn-comment-collapse-label::after {
+    content: "+";
+  }
 }
 
 .is-comment-collapsed:not(:target) {

tildes/static/js/behaviors/comment-collapse-button.js

@@ -14,12 +14,6 @@ $.onmount("[data-js-comment-collapse-button]", function() {
             $comment.toggleClass("is-comment-collapsed");
         }
 
-        if ($comment.hasClass("is-comment-collapsed")) {
-            $this.text("+");
-        } else {
-            $this.html("−");
-        }
-
         $this.blur();
     });
 });

tildes/tildes/templates/macros/comments.jinja2

@@ -45,7 +45,7 @@
   <div class="comment-itself">
     <header class="comment-header">
       <button class="btn btn-light btn-comment-collapse" data-js-comment-collapse-button>
-        {{ "+" if comment.collapsed_state in ("full", "individual") else "−" }}
+        <span class="btn-comment-collapse-label"></span>
       </button>
 
       {% if not is_individual_comment and comment.num_children > 0 %}