Documenting instructions for attachments.

Also some document hierarchy changes to try to make the next steps more prominent.
Add Optimizely hook to page edit button

Optimizely can track this button by ID, class name, or complex selector.
All of these change from time to time, however, so having a dedicated
attribute will be ideal. It is also self-documenting, making it clear to
readers that the attribute cannot change without coordination with
Optimizely test managers.
Merge pull request #2496 from stephaniehobson/docs-install-attachments

Documenting instructions for attachments.
Merge pull request #2493 from openjck/optimizely-edit-button-hook

Add Optimizely hook to page edit button
Rename some GA event categories, actions, labels

Luke decided on these new names as part of ongoing event cleanup work.
Merge pull request #2495 from openjck/rename-ga-events

Rename some GA event categories, actions, labels
fix bug 1002043 - Allow consecutive underscores in slugs
Remove double jQuery usage in components
Fix save buttons overlapping with title field on wiki edit page
Updated chief_deploy.py and update_site.py to do the same migration dance as the Puppet manifest.

- The update to chief_deploy.py needs to be changed to have the commit id of the commit that is going to be deployed. So probably not the merge commit, but the last commit shown in the pull request.
- The update to update_site.py needs to be removed right after merging the pull request.
Make sure the migration can be executed by depending on the latest available migration in waffle.
Make sure we install always the latest 1.4.x version in Vagrant.
Move events app into own app under the new kuma namespace.
Moved user profile model from devmo into users app.

- This also gets rid of lots of unused/unwanted code in the users app, e.g. email change model.
- Moved over tests and fixtures.
- Combines URL patterns of users and profile views.
Fixed templates after app move.

- deletes lots of unused template files
- removes links to removed views, e.g. old register view
- adds a simplified way to refer to user profile URLs `{{ <user>.get_absolute_url() }}`
Added initial events migration. This needs to be faked.
Removed all the devmo migrations and add users app migrations.
Minor cosmetic changes.
Removed unneeded rendering of the send_email_reminder form as it doesn't make sense anymore.
Use correct form to show errors for.