E-learning: Add courses routes, index, and new form

Feature: Courses
As a site teacher
In order to be able to teach things
I would like to create courses

Scenario: Creating a new course
Given I have logged in
And I am on the "courses" page
When I click the "New Course" button
Then I should see "Creating a new Course"
And I should see a form with:
| Field |
| Title |
| Description |
| Slack channel name |

Snippets (if we decide to use simple_form gem):

// Gemfile
gem 'simple_form'

// Console
bundle install
rails generate simple_form:install

// Any form can look something like
= simple_form_for @course do |f|
= f.error_notification
= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present?
= f.input :title
= f.label :description
= f.rich_text_area :description
/= f.submit 'Save'
= f.button :submit