Replaced all occurrences of "iconURL" in the extension's repo with
"iconUrl" after spotting a typo in `mw.echo.dm.NotificationItem.js#13`,
"iconUrl" (find the correct "iconURL" in `mw.echo.Controller.js#394` in
the diff). Thankfully the typo was only in the documentation block, but
given the casing of other config object properties like "primaryUrl"
and "secondaryUrl", it would be best to make them all consistent to
prevent any bug that could happen in the future.
Change-Id: I56e6a1d2c7695204b35e767679a27ee22b3fe4bc
