Add Webhooks for Releases

What does this MR do?

This MR add the ability to receive release events (created + updated) for project and group webhooks.

Note: I was unable to test the group webhooks frontend due to no EE license.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #26873 (closed)

Database migrations:

Rollback procedure:

rake db:migrate:down VERSION=20201008075620

rake db:migrate:up log:

== 20201008075620 AddReleasesEventsToWebHooks: migrating ======================
-- add_column(:web_hooks, :releases_events, :boolean, {:null=>false, :default=>false})
   -> 0.0025s
== 20201008075620 AddReleasesEventsToWebHooks: migrated (0.0025s) =============

rake db:migrate:down log:

== 20201008075620 AddReleasesEventsToWebHooks: reverting ======================
-- remove_column(:web_hooks, :releases_events, :boolean, {:null=>false, :default=>false})
   -> 0.0017s
== 20201008075620 AddReleasesEventsToWebHooks: reverted (0.0051s) =============