fix(list-selection): Properly iterate over the selected indexes when expanding selection

This commit fixes a bug in which you couldn't shift select threads from
the bottom to the top (i.e. select a thread, shift-click a thread on top of it
would not select the range of threads).

This bug was introduced by a translation from coffeescript to JS.
Specifically, by converting

```
for idx in [startIdx..endIdx]
```

to
```
for (let idx = startIdx; idx <= endIdx; idx++) { ... }
```

The coffeescript range syntax for `[x..y]` automatically generates the
correct range when x > y or when x < y, generating a descending or
ascending range respectively.
However, our transaltion simply iterated ascendingly from `startIdx`
to `endIdx`, which would be a no-op when the `startIdx` was greater than
the `endIdx`, which was the case when shift-selecting threads from bottom to top.

src/components/list-selection.es6

@@ -145,11 +145,15 @@ export default class ListSelection {
       if (startIdx === -1 || endIdx === -1) {
         return;
       }
-      for (let idx = startIdx; idx <= endIdx; idx++) {
+      const count = Math.abs(startIdx - endIdx) + 1
+      const indexes = new Array(count)
+        .fill(0)
+        .map((val, idx) => (startIdx > endIdx ? startIdx - idx : startIdx + idx))
+      indexes.forEach((idx) => {
         const idxItem = this._view.get(idx);
         this._items = _.reject(this._items, t => t.id === idxItem.id);
         this._items.push(idxItem);
-      }
+      })
     }
     this.trigger();
   }