Remember last app that was pushed to and warn when using a different one

[Hades32]

Expected Behavior

~/my-app1 $ balena push my-app1
# pushing...
~/my-app1 $ balena push some other-app
You pushed to "my-app1" before, do you really want to change to "other-app" [y/n] Y
~/my-app1 $ balena push other-app
# pushing...

Motivation

There is currently no way to prevent a user from accidentally pushing to the wrong app. Using Bash auto-complete makes it likely to to hit this.

After we have this we could make it even as easy as doing balena push, omitting the app name the second time

Additional Notes

Discussed in Product Call 2021-04-01: https://jel.ly.fish/359bd49b-d421-4072-a4d8-bff21ccb06eb

.balenarc might be a good place to store this state as it can be added to .gitignore