WYSIWYG: Fixed growing rows on Firefox

Occured when the cell contained any block content with a differnt line
height to the table cell itself.
In firefox, cells with a height would end up with an actual greater
real cell height, which messed up TinyMCE resize calculations, causing
tables to grow.
Adding default vertical-align: top, changes this behaviour to get proper
cell heights.
Related to Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=569645
Have tested that editor cell text align options can still be used with
this.

For #4337

resources/sass/_tables.scss

@@ -12,6 +12,7 @@ table {
     overflow: auto;
     line-height: 1.2;
 	word-break: break-word;
+    vertical-align: top; // Workaround for: https://bugzilla.mozilla.org/show_bug.cgi?id=569645
   }
   td p, th p {
     margin: 0;