Add application setting for What's new [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

There's the What's new feature:

With each monthly release, GitLab includes some of the highlights in the What's new feature. To access it, select the ? icon and select What's new from the menu.

Regarding the What's new feature, two wishes were expressed in issues:

• #325151 (closed): It should be possible to disable this feature.
• #322862 (closed): Only new features for the current tier should be displayed. New features of higher tiers should be hidden.

So this MR implements a new application setting (Admin Area > Settings > Preferences > What's new) to choose between three variants:

• Enabled: all tiers (default)
• Enabled: current tier only
• Disabled

Resolves #325151 (closed) #322862 (closed)

Screenshots

Admin Area > Settings > Preferences > What's new

What's new drawer

	Enabled: all tiers	Enabled: current tier only	Disabled
Menu
Drawer (current tier: Free)	new features for non-free tiers also shown:	new features for non-free tiers hidden:	no access

🛠 with ❤ at Siemens

/cc @bufferoverflow

Database Review

DB Migration: Output

rails db:migrate VERSION=20210407111027 RAILS_ENV=development
== 20210407111027 AddWhatsNewApplicationSetting: migrating ====================
-- add_column(:application_settings, :whats_new_variant, :integer, {:limit=>2, :default=>0})
   -> 0.0120s
== 20210407111027 AddWhatsNewApplicationSetting: migrated (0.0122s) ===========

DB Rollback: Output

rails db:rollback STEP=1  RAILS_ENV=development
== 20210407111027 AddWhatsNewApplicationSetting: reverting ====================
-- remove_column(:application_settings, :whats_new_variant, :integer, {:limit=>2, :default=>0})
   -> 0.0087s
== 20210407111027 AddWhatsNewApplicationSetting: reverted (0.0118s) ===========

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a 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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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