Make fingerprints selectable

Fixes #500
Closes #501

// FREEBIE

stylesheets/_conversation.scss

@@ -52,6 +52,7 @@
   .key {
     font-family: monospace;
     padding: 0 1em;
+    -webkit-user-select: text;
   }
 }
 

stylesheets/manifest.css

@@ -620,7 +620,8 @@ input.search {
     padding: 1em; }
   .key-verification .key {
     font-family: monospace;
-    padding: 0 1em; }
+    padding: 0 1em;
+    -webkit-user-select: text; }
 
 .message-detail .container {
   height: calc(100% - (36px + 4px)); }