Builder title gets a character count view (#562).

In builder:

- Take baked-in character count behavior out of RichTextEditorMixin.
- Make CharacterCountView, a reusable view that tracks character
  counts in inputs and wysihtml5 editors.
- Rename RichTextEditorMixin.getEditor to createEditor. Add a
  getEditor method that just returns the created editor.
- Update default warning text of character count view to
  "Shorter is better!"
- Give the summary section title a character count, with a limit of
  100 characters.
- Update copy in placeholder of summary title text.
Integration test scripts for user registration

Addresses #786
Fix import error related to django-social-auth

The exception classes seem to have gotten moved between different
versions of django-social-auth.  Add a try/except around the import
to support the different locations.
Support django-registration 1.0

With version 1.0, django-registration switches to using class-based
views rather than function-based views and backend classes. This
commit updates our registration customizations to work with
class-based views.

Addresses #786
Builder character count view refinements (#562).

- Remove unnecessary _.defaults call.
- Upgrade to jquery.tooltipster 2.1.4.
- Use a tooltip rather than inline text to display warning about
  over-character-limit inputs.
- Stop polling when sections change--don't wait for a blur event.
- Add a warning-color set to variables.less.
- Use warning colors and icon to highlight over-limit character
  counts.
- Prevent input elements from moving in summary section when
  character count is revealed.
Remove bold weight in builder summary title (#562).
More clear activation failure message

When a new account activation fails, include suggestions of possible
reasons for the failure including miscopying the link and having
already clicked the link.

Addresses #836
Clean up user registration test scenarios

Addresses #786
Updated copy for activation failure page

Made the top-level page title shorter, made the causes of this error a
bulleted list, and added container elements and classes to improve
formatting.

Addresses #836
Remove old version (1.2) of tooltipster (#562).
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
Show a link to the builder when there are no stories

When a user hasn't authored any stories, show a link to the builder in
the "My Stories" view.

Closes #840
Merge pull request #839 from patternleaf/562-story-title-char-count

Merge implementation for issue #562: Story builder should issue a notification for long titles
Update changelog for #562
Fix up "Save" button last-saved tooltip

Fix updating the "Save" button tooltipster-generated tooltips.

* Call 'update' when the last-saved timestamp is updated
* Only update the title attribute if tooltipster is not present, avoiding
  redundant tooltips
* Hide the tooltip when the button is clicked
* Only check if the tooltipster plugin is present once, rather than with
  each call

Closes #841