Use <td class="n" data-n="3"> instead of <th>3</th> for line numbers

Summary:
Ref T13161. Ref T12822. See PHI870. Long ago, the web was simple. You could leave your doors unlocked, you knew all your neighbors, crime hadn't been invented yet, and `<th>3</th>` was a perfectly fine way to render a line number cell containing the number "3".

But times have changed!

  - In PHI870, this isn't good for screenreaders. We can't do much about this, so switch to `<td>`.
  - In D19349 / T13105 and elsewhere, this `::after { content: attr(data-n); }` approach seems like the least bad general-purpose approach for preventing line numbers from being copied. Although Differential needs even more magic beyond this in the two-up view, this is likely good enough for the one-up view, and is consistent with other views (paste, harbormaster logs, general source display) where this technique is sufficient on its own.

The chance this breaks //something// is pretty much 100%, but we've got a week to figure out what it breaks. I couldn't find any issues immediately.

Test Plan:
  - Created, edited, deleted inlines in 1-up and 2-up views.
  - Replied, keyboard-navigated, keyboard-replied, drag-selected, poked and prodded everything.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13161, T12822

Differential Revision: https://secure.phabricator.com/D20188

webroot/rsrc/js/application/diff/DiffChangesetList.js

@@ -70,13 +70,13 @@ JX.install('DiffChangesetList', {
     var onrangedown = JX.bind(this, this._ifawake, this._onrangedown);
     JX.Stratcom.listen(
       'mousedown',
-      ['differential-changeset', 'tag:th'],
+      ['differential-changeset', 'tag:td'],
       onrangedown);
 
     var onrangemove = JX.bind(this, this._ifawake, this._onrangemove);
     JX.Stratcom.listen(
       ['mouseover', 'mouseout'],
-      ['differential-changeset', 'tag:th'],
+      ['differential-changeset', 'tag:td'],
       onrangemove);
 
     var onrangeup = JX.bind(this, this._ifawake, this._onrangeup);
@@ -360,7 +360,7 @@ JX.install('DiffChangesetList', {
           while (row) {
             var header = row.firstChild;
             while (header) {
-              if (JX.DOM.isType(header, 'th')) {
+              if (this.getLineNumberFromHeader(header)) {
                 if (header.className.indexOf('old') !== -1) {
                   old_list.push(header);
                 } else if (header.className.indexOf('new') !== -1) {
@@ -1247,11 +1247,7 @@ JX.install('DiffChangesetList', {
     },
 
     getLineNumberFromHeader: function(th) {
-      try {
-        return parseInt(th.id.match(/^C\d+[ON]L(\d+)$/)[1], 10);
-      } catch (x) {
-        return null;
-      }
+      return parseInt(th.getAttribute('data-n'));
     },
 
     getDisplaySideFromHeader: function(th) {