Make last update check persistent

[vcsjones]

Today, the last update check is not persistent. That is, it checks once when the application starts up, and every day thereafter (under normal circumstances) so long as the app is running.

If you are frequently starting and stopping Fiddler, this can cause you to hit GitHub's API limit kinda quick, especially if you are on a shared corporate IP.

We should instead keep the last update check in configuration somewhere so the data survives application restarts.