Updated changelog icons.
Removed the mainview code which was used by the old sidebar
Merge pull request #388 from huwshimi/changelog-icons

Updated changelog icons

Added the new icon assets as provided by design.
Merge pull request #389 from hatched/remove-old-code

Removed the mainview code which was used by the old sidebar

All of the functionality that was in mainview was moved to the dispatcher methods so its no longer needed. It appears that there were no test which tested this code directly, only as a side effect of the browser.js instantiating it so those tests are still valid with the new setup.

#### To QA
Do an exploratory QA on items involving the sidebar. Everything should work as expected.