Updates plugin to 2.2.0

**NOTE: this PR incorporates all of the other open PRs right now**

With this PR, we merge in a large number of fixes and changes:

* Feature: adds shortcode to easily render login button
* Feature: use modal to allow users to login and preserve state when they are logged out with Clef
* Feature: menu notification when user hasn't configured Clef
* Feature: by default, embed the Wave in the login form rather than making users click a button
* Fix: issue where if passwords were fully disabled, but Clef wasn't configured, no login form would be shown
* Fix: issue where a new session was opened for every request, not just ones where it was necessary (in admin for Clef users)
* Fix: issue where users can connect the same Clef account to two WordPress accounts
* Fix: issue where if the heartbeat API wasn't available, Clef could interfere with the loading of other plugins
* Fix: issue where prompt to add Clef badge was shown even if the badge was already displayed

Any feedback and testing, as always, is much appreciated.