Friendlier, shorter account activation failure message

Remove word "failure".

Emphasize actions over causes.

Remove lengthy descriptions of possible causes.

Addresses #836
Activation view refinements

* Log activation failures
* Flash a success message using the messages framework
* Don't automatically login the user
* Redirect to the login page

Addresses #836
Connect login link on activation failure page

This got accidently removed in 8e032a4b74d20249050103ec4f1c5438ba626e3a

Addresses #836
Update changelog for #836