Ember tries to reuse DOM elements when it can but as ember looks for
changes to objects rather than the DOM itself sometimes. This and the
fact that an objects identity may change even though its value hasn't,
results in ember occasionally re-mutating DOM when it doesn't need to.
The `each` helper includes a `key` attribute to hint to ember what it
should look for when deciding whether something has changed, rather than
the objects identity.
https://api.emberjs.com/ember/release/classes/Ember.Templates.helpers/methods/each#specifying-keys
We use this here to fix an issue where DOM was being redrawn after the
user had scrolled the page and was therefore resetting the scroll back
to 0 (the top of the page)
e4a00b39bf