UX: Use flat button style for bulk selection

app/assets/javascripts/discourse/templates/flat-button.raw.hbs

@@ -0,0 +1,3 @@
+<button class='btn-flat {{class}}' title='{{i18n "topics.bulk.toggle"}}'>
+  {{fa-icon icon}}
+</button>

app/assets/javascripts/discourse/templates/topic-list-header-column.raw.hbs

@@ -1,7 +1,7 @@
 <th data-sort-order='{{order}}' class='{{view.className}}'>
   {{~#if canBulkSelect}}
     {{~#if showBulkToggle}}
-        <button class='btn bulk-select' title='{{i18n "topics.bulk.toggle"}}'><i class='fa fa-list'></i></button>
+      {{raw "flat-button" class="bulk-select" icon="list" title="topics.bulk.toggle"}}
     {{/if ~}}
     {{~#if bulkSelectEnabled}}
       <span class='bulk-select-topics'>

app/assets/javascripts/discourse/templates/topic-list-header.raw.hbs

@@ -1,7 +1,7 @@
 {{#if bulkSelectEnabled}}
   <th class='star'>
     {{#if canBulkSelect}}
-      <button class='btn bulk-select' title='{{i18n "topics.bulk.toggle"}}'><i class='fa fa-list'></i></button>
+      {{raw "flat-button" class="bulk-select" icon="list" title="topics.bulk.toggle"}}
     {{/if}}
   </th>
 {{/if}}

app/assets/stylesheets/common/components/buttons.scss

@@ -194,4 +194,7 @@
   background: transparent;
   border: 0;
   outline: 0;
+  i.fa {
+    opacity: 0.7;
+  }
 }

app/assets/stylesheets/desktop/topic-list.scss

@@ -61,10 +61,8 @@
   }
   button.bulk-select {
     padding: 0;
-    background: transparent;
-    &:hover {
-      color: $primary-low; 
-    }
+    font-size: 1.05em;
+    margin-right: 0.3em;
   }
 
   .star {