Make retry intervals persist across browser sessions

[Alexei95]

In my opinion it would be nice if there were a checkbox to enable saving the reload intervals of tabs across browsing sessions, because personally I have multiple tabs with different reload intervals, and setting them up again at each startup is a bit painful, so having it done directly by the add-on would be much better.

[blackwind]

This is something I had already intended to look at for the next major version. No promises, but we'll see.