Add Rails/ReversibleMigration cop

[sue445]

This cop checks whether the #change method of the migration file is reversible.

Example

# bad
def change
  change_table :users do |t|
    t.column :name, :string
  end
end

# good
def change
  create_table :users do |t|
    t.string :name
  end
end

# good
def change
  reversible do |dir|
    change_table :users do |t|
      dir.up do
        t.column :name, :string
      end

      dir.down do
        t.remove :name
      end
    end
  end
end

drop_table

# bad
def change
  drop_table :users
end

# good
def change
  drop_table :users do |t|
    t.string :name
  end
end

change_column_default

# bad
def change
  change_column_default(:suppliers, :qualification, 'new')
end

# good
def change
  change_column_default(:posts, :state, from: nil, to: "draft")
end

remove_column

# bad
def change
  remove_column(:suppliers, :qualification)
end

# good
def change
  remove_column(:suppliers, :qualification, :string)
end

remove_foreign_key

# bad
def change
  remove_foreign_key :accounts, column: :owner_id
end

# good
def change
  remove_foreign_key :accounts, :branches
end

Note

reversible migration commands are below.
http://api.rubyonrails.org/classes/ActiveRecord/Migration/CommandRecorder.html

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

That's one really handy cop! I'll review the code a bit later. It'd be nice if you wrote a matching rule for the Rails Style Guide.

[bbatsov]

Seems you forgot to update the documentation.

[sue445]

@bbatsov I updated doc and CI is all green 💚

It'd be nice if you wrote a matching rule for the Rails Style Guide.

I see, I'll write a style guide later. I temporarily make this PR to WIP

[sue445]

@bbatsov I wrote style guide at rubocop/rails-style-guide#211 , and removed WIP. Please check these 🙇

[sue445]

🎉