Provide styling on issues that user has not looked at since last update

The use case is that when reviewing code you don't want to keep looking at it unless someone has changed it since the last time you went through it.

Idea 1:

* Store the last viewed time for every issue in local storage
* When showing an issue, style it based upon the last update time vs last viewed
* Update last viewed time when showing details or opening link about issue

This works very well and is fast, but has problem that it is tied to the local browser.  In my opinion this is a reasonable tradeoff.

Idea 2:

Same as above but track comment count.  This should work better and give more specific results.

You should look again when one of these changes:
* new comments
* new pull comment
* I was mentioned (no idea how to implement)
* change hash of head
* change in description
* change in title

<!-- TRESTLE
{"weight":0,"columnWeight":0,"milestoneWeight":-246}
-->