Simplify styling tags

This way we won't have to add the `inline-block` and `no-bullet` classes
to other elements and we can define the styles in one place.

Note we're using the `ul.tags` selector instead of just `.tags` to avoid
conflicts with a `div.tags` selector which is used to select tags in a
form. Renaming the selector to `.tag-list` would be better, but we
aren't doing so because it would break custom stylesheets using that
selector.

Also note we're keeping the %tags placeholder selector in `.tags`
selector in the participation.scss file. This is so styles are not
overwritten by selectors like `.debate-show ul li`, which has the same
specifity as `ul.tags li`.

app/assets/stylesheets/participation.scss

@@ -408,10 +408,10 @@
     }
   }
 
-  .tags,
   .geozone {
 
     li {
+      display: inline-block;
       margin-bottom: 0;
     }
   }
@@ -487,8 +487,7 @@
   }
 
   .tags {
-    display: block;
-    margin-bottom: 0;
+    @extend %tags;
 
     a {
       margin-right: rem-calc(6);
@@ -673,8 +672,7 @@
   }
 
   .tags {
-    display: block;
-    margin-bottom: 0;
+    @extend %tags;
 
     a {
       font-size: $tiny-font-size;