Add login

This changeset adds authentication with Devise, which is necessary now for KriKri.  It also installs KriKri and Blacklight, the result of `rails generate krikri:install`.

See: https://www.pivotaltracker.com/n/projects/1172184/stories/80648080

I'm also about to issue a pull request for the KriKri topic branch `add-logins`.  I'll follow up with a comment in this thread giving its URL.

With these changes, you should be able to check out this branch and install a very skeletal Heiðrún, but one with login pages:

```
bundle install
bundle exec db:migrate
```

Note the comments in 19d93373 and 32f6ef92