First step to remove stateful-button.

Introduce button-ajax helper for migrating uses of stateful-button.
Show how to use it by switching env-var creation to use managed-button.
managed-button for github invites
managed-button for rebuilding buttons
managed-button for cancel
use managed-button in project settings forms

and common header for follow-project
managed-button for extend-trial-clicked

It was already using release-button
managed-button for enable-notice
managed-button for followed-repo

This still has two problems:

1) the api-data-count=2 thing is gone, so the button quickly shows
   "unfollow" before redirecting when should-build?=true
2) if you follow then unfollow (or vice versa) the spinner gets stuck

Both problems will be fixed in a subsequent commit.
removed now dead stateful-button
Simplify managed-button

Fixes add-projects button behaviors too.
fixup! use managed-button in project settings forms
do not close! in button-ajax
Fix button idle bug.
Merge commit '480644d83b749c1a432f4559e7e0a7b94541e88d'