AttributeManager: Fix bogus `this` during attribute removal

Before this commit, the callback passed to `.map()` during attribute
removal was a normal function, not an arrow function. This meant that
the value of `this` in the function body depended on how the callback
was invoked. In this case, the callback was invoked without any
explicit context (it was not called as a method, nor was it called via
`.call()`, `.apply()`, or `.bind()`). Without any explicit context,
the value of `this` depends on strict mode. Currently the function is
in sloppy mode, so `this` refers to the "global this" object (a.k.a.,
`window`). It doesn't make sense for the callback to reference
`window.author`, so I'm assuming the previous behavior was a bug.

Now the function is an arrow function, so the value of `this` comes
from the enclosing lexical context, which in this case is the
AttributeManager object. I believe that was the original intention.

src/static/js/AttributeManager.js

@@ -349,7 +349,7 @@ AttributeManager.prototype = _(AttributeManager.prototype).extend({
     const hasMarker = this.lineHasMarker(lineNum);
     let found = false;
 
-    const attribs = _(this.getAttributesOnLine(lineNum)).map(function (attrib) {
+    const attribs = _(this.getAttributesOnLine(lineNum)).map((attrib) => {
       if (attrib[0] === attributeName && (!attributeValue || attrib[0] === attributeValue)) {
         found = true;
         return [attributeName, ''];