Misc cleanups & changes, feel free to pick and choose

Hi. I've been using streak for a while as a low-barrier way to record my runs. I ran into a problem with OAuth tokens expiring if I went for two days without a streak -- I had to manually rm the cache file every time -- and ended up futzing with the refresh logic. It now seems to work, whether the token needs refreshing or full reauth.

Then, there's a commit that allows using non-public OAuth secrets, because I just didn't know enough of OAuth to trust my calendar to an app that has its secrets on Github.

And then I moved the cache file under ~/.cache, because that seems to be the convention and it's nice to have all always-removable files under one path.

Feel free to cherry-pick and rebase as much as you want.