Autoactivate inactive users on external_auth login

Conditional on existing flag BYPASS_ACTIVATION_EMAIL_FOR_EXTAUTH

This improves the behavior of external_auth logins (mainly motivated by Shib).  Shib tries to find and link existing accounts when users first use it to login.  The previously behavior had not auto-activated linked-users that weren't active (even though there was a FEATURE flag that specifies this), which caused some number of support emails.  This PR now has external_auth activating inactive users upon login (when the flag is set), which covers the previously missing case.

@singingwolfboy you can send this PR wherever appropriate, but @carsongee @brianhw and @ormsbee are familiar with this code.