Move comment and thread elements to shared

Even before the refactorings, the comment and thread elements were also used
from views outside of diff/. Now they are now completely independent from
gr-diff and descendants, and can move into their own folder and drop the -diff
from their name.

The dependency left from diff/gr-diff-host onto comment[-thread], makes sense
since gr-diff-host is the Gerrit wrapper for gr-diff with gr-comment[-thread].

Change-Id: I5076428da980198989edc605e5edc0e4d66529dd
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
index 53a60d7..ab9daec 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.html
@@ -256,7 +256,7 @@
       threadEls[0].dispatchEvent(
           new CustomEvent('thread-discard', {detail: {rootId: 4711}}));
       const attachedThreads = element.queryAllEffectiveChildren(
-          'gr-diff-comment-thread');
+          'gr-comment-thread');
       assert.equal(attachedThreads.length, 1);
       assert.equal(attachedThreads[0].rootId, 42);
     });
@@ -1130,7 +1130,7 @@
           commentSide, undefined, false));
 
       let threads = Polymer.dom(element.$.diff)
-          .queryDistributedElements('gr-diff-comment-thread');
+          .queryDistributedElements('gr-comment-thread');
 
       assert.equal(threads.length, 1);
       assert.equal(threads[0].commentSide, commentSide);
@@ -1151,7 +1151,7 @@
           '3', 1, commentSide, range, true));
 
       threads = Polymer.dom(element.$.diff)
-          .queryDistributedElements('gr-diff-comment-thread');
+          .queryDistributedElements('gr-comment-thread');
 
       assert.equal(threads.length, 2);
       assert.equal(threads[1].commentSide, commentSide);