Add a notify action to the notifications component

Refs #3160
- gh-notifications component now takes an optional notify
  parameter.  If present it will be invoked as an action
  when a notification is added or removed.
- Add a data-notification-count attribute to the main container
  that tracks the number of "top" notification messages that
  are currently being shown.
Extending context concept to models

fixes #3275, fixes #3290, ref #3086, ref #3084

- Ensure that we use the current logged in user and not just user 1 when
- removing hard coded user: 1 except where absolutely necessary
- passing context, rather than user to models
- base model has a new function to determine what id to use for created_by etc
Prevent duplicate slug request on saving new post

No Issue
- Do not run generateSlugPlaceholder if save has been initiated
  and the title has already been set on the post. At that stage
  a slug has already been generated and another API call is not
  necessary.
Wrap up schema amends for 003

closes #2600

- adds 2 new columns to tags, for the soon-to-land tag management UI
- fixes validation for the active bool in app_fields
- adds missing return to addColumn command
- cleanup in addTableColumn util
Check setup status when making API responses

Closes #3303, Closes #3299
- Check whether or not setup has been completed when deciding
  how to respond to certain API requests.
- Add tests.
Merge pull request #3314 from jaswilli/issue-3160

Add a notify action to the notifications component
Merge pull request #3319 from ErisDS/issue-3275

Extending context concept to models
Merge pull request #3320 from jaswilli/dup-slug-get

Prevent duplicate slug request on saving new post
Merge pull request #3321 from ErisDS/issue-2600-fin

Wrap up schema amends for 003
Add Author dropdown to PostSettingsMenu

Ref #3084
This PR does NOT hide the dropdown as required to close 3084.

- `EditorNewRoute` creates post with the author set to the current user
- added `authors` ArrayPromiseProxy (whoa, what?) to PSM
- added `changeAuthor` function that sets author and saves model when the user selects a new author
Merge pull request #3322 from jaswilli/check-setup

Check setup status when making API responses
Merge pull request #3324 from novaugust/psm-authors

Add Author dropdown to PostSettingsMenu
Model & test cleanup