undomodule: disallow undoing "clear authorship colors"

Clearing the authorship colors of a document with at least two authors, and then
undoing that action caused a disconnect from the pad.
This change disallows undoing clearing authorship colors in order to prevent
the problem from affecting users, and adds the relative test coverage.

This is a change of behaviour, and is documented in the changelog.

Fixes #2802 (sidestepping it).

src/static/js/Changeset.js

@@ -791,6 +791,9 @@ exports.textLinesMutator = function (lines) {
         }
       } else {
         var sline = putCurLineInSplice();
+        if (!curSplice[sline]) {
+          console.error("curSplice[sline] not populated, actual curSplice contents is ", curSplice, ". Possibly related to https://github.com/ether/etherpad-lite/issues/2802");
+        }
         curSplice[sline] = curSplice[sline].substring(0, curCol) + text + curSplice[sline].substring(curCol);
         curCol += text.length;
       }

src/static/js/ace2_inner.js

@@ -292,6 +292,7 @@ function Ace2Inner(){
   {
     if ((typeof author) != "string")
     {
+      top.console.error("Going to throw new error, potentially caused by: https://github.com/ether/etherpad-lite/issues/2802");
       throw new Error("setAuthorInfo: author (" + author + ") is not a string");
     }
     if (!info)

src/static/js/undomodule.js

@@ -253,7 +253,15 @@ var undoModule = (function()
       }
       if (!merged)
       {
-        stack.pushEvent(event);
+        /*
+         * Push the event on the undo stack only if it exists, and if it's
+         * not a "clearauthorship". This disallows undoing the removal of the
+         * authorship colors, but is a necessary stopgap measure against
+         * https://github.com/ether/etherpad-lite/issues/2802
+         */
+        if (event && (event.eventType !== "clearauthorship")) {
+          stack.pushEvent(event);
+        }
       }
       undoPtr = 0;
     }