Welcome Tour: Update the user model

[ErisDS]

This issue is part of the welcome tour feature and should be implemented behind a config flag (not labs) to start with. An example of how to do this exists in this PR.

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Description:

On the server side, the user model should get a single additional field which stores a JSON object containing each tour item and whether or not it is seen. This data will be passed via the User API as per usual, such that Ember always knows the state of the currently logged in user's tour.

Spec:

The user model needs a new field added to it. Recommended name is tour, and it should have the same type and maxlength as value in the settings table. The tour field should be nullable and not have a default value, tour items will appear there when they have some data to persist (see #).

Adding new columns to our database schema in schema.js means we need to ensure that a migration is triggered. This means we also need to bump our database version in default-settings.json and update the hashes in the tests that check this doesn't change.

It will also be necessary to update several API tests and other locations to ensure that the API knows about and properly handles the new tour field.

Note: it would be ideal to bundle the additional post field needed for the preview feature into this database migration.

The result of this issue should be the new tour field being provided via the User API.

[letsjustfixit]

@ErisDS

[..] we'll need to add a new preview field to the posts table which defaults to private. This field would
be used when handling the /p/ route [..]

Could you please clarify the max size of the preview field mentioned above? The only specific there was sebgie's 20 chars i could find.

For now its:
preview: {type: 'string', maxlength: 20, nullable: true, defaultTo: 'private'},

[ErisDS]

It no longer makes sense to bundle the changes together. As explained in the preview issue, we aren't going to be adding the part of the feature that needs it for a while.