Change the content plans permissions so that everyone can create and edit plans

https://www.pivotaltracker.com/story/show/68637746
Only allow plans to be destroyed when they are empty

https://www.pivotaltracker.com/story/show/68637746
Add a better unauthorised message

At the moment it's a rather cryptic pundit error.

Also hides the delete button if there are plans associated.
Travis configs
do not use coffeescript
avoid 500 when `organisation_brand_colour_class_name` is nil
add `gds_editor` trait to `user` factory

DRY
enhance MockNeedsApi

it mocks now one need by default
enhance content_plan factory and add tags factory
content plan filter specs
fix deprecation warning

fixes the following:
------

DEPRECATION WARNING: It looks like you are eager loading table(s) (one of: content_plans, content_plan_taggings_947d740, content_plan_needs, content_plan_users) that are referenced in a string SQL snip
pet. For example:

    Post.includes(:comments).where("comments.title = 'foo'")

Currently, Active Record recognizes the table in the string, and knows to JOIN the comments table to the query, rather than loading comments in a separate query. However, doing this without writing a full-blown SQL parser is inherently flawed. Since we don't want to write an SQL parser, we are removing this functionality. From now on, you must explicitly tell Active Record when you are referencing a
 table from a string:

    Post.includes(:comments).where("comments.title = 'foo'").references(:comments)

If you don't rely on implicit join references you can disable the feature entirely by setting `config.active_record.disable_implicit_join_references = true`. (called from _app_views_content_plans_index
_html_slim___4490503596098976456_69909357144160 at /var/govuk/content-planner/app/views/content_plans/index.html.slim:30)
show organisations on content_plan#show
specs for content plan
refactor validations
factories for content
specs for Content#percentages_for

and minor refactoring on `Content`
specs for content_plans#show
show organisations in `Content`
add `Content` specs

and refactor `ContentPlan` specs to use `CommonSteps` (shared with `ContentSteps`)
refactor `content_plan_filter_spec` into `content_plan_spec`