Utilize `json_fields` to expose fields via
`Service#as_json(only: json_fields)`.

This commit removes one RuboCop offense from its todo file.

Creating new records has been disabled, and all
existing records been migrated to clusters as of
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28534

- instead of using properties
- backward compatibility has to be kept for now

- create tables for storing issue trackers properties
- add model and basic logic & spec

This enables sending a chat message to Slack or Mattermost
  upon a successful, failed, or canceled deployment

This reverts commit a5378665.

Fixes gitlab-org/gitlab-ce#42595

Partially addresses #47424.

`nil` value is not included in
`ActiveModel::Type::Boolean::FALSE_VALUES` which caused that in Rails 5
the boolean_accessor converted `nil` to `true` instead of `false`.

In Rails 5.0 the `ActiveRecord::ConnectionAdapters::Column::TRUE_VALUES`
constant has been removed [1] and the remaining `FALSE_VALUES` constant
has been moved to `ActiveModel::Type::Boolean` [2]

[1]: https://github.com/rails/rails/commit/a502703c3d2151d4d3b421b29fefdac5ad05df61
[2]: https://github.com/rails/rails/commit/9cc8c6f3730df3d94c81a55be9ee1b7b4ffd29f6

[10.6] Prevent notes on confidential issues from being sent to chat

See merge request gitlab/gitlabhq!2366
# Conflicts:
#	app/helpers/services_helper.rb

Removes confusing/unnecessary checkboxes when trying to configure an
integration. If there is only one supported event we don't need to
allow these to be individually disabled since the integration can be
disabled instead.

E.g. Project Integrations for GitHub, Bugzilla, Asana, Pipeline emails and Gemnasium

Allows integrations to override which triggers are configurable