UsagePing EE features frontend

What does this MR do?

Relates to #333018 (closed), #333022 (closed), #333021 (closed)

Adds frontend UI changes for enabling Registration features (EE features) if usage ping is enabled.

For Database:

Adds a boolean column to check if the feature is enabled via checkbox. The column is added in ApplicationSettings table.

As part of the mentioned issues, we are offering our free self-managed customers one paid feature (Email from Gitlab) in exchange for enabling UsagePing. This MR adds changes for showing that option to enable the feature, uncheck the checkbox if UsagePing is disabled. This MR also adds a database column named usage_ping_features_enabled in ApplicationSettings table to store if the admin has enabled it or not, after enabling UsagePing

Screenshots (strongly suggested)

UsagePing disabled state

UsagePing enabled state

DB Migrations

UP migrations

rake db:migrate

== 20210623074226 AddUsagePingFeaturesEnabledToApplicationSettings: migrating =
-- add_column(:application_settings, :usage_ping_features_enabled, :boolean, {:default=>false, :limit=>nil, :null=>false})
   -> 0.0105s
== 20210623074226 AddUsagePingFeaturesEnabledToApplicationSettings: migrated (0.0106s)

Down migrations

rake db:migrate:down VERSION=20210623074226

== 20210623074226 AddUsagePingFeaturesEnabledToApplicationSettings: reverting =
-- remove_column(:application_settings, :usage_ping_features_enabled)
   -> 0.0116s
== 20210623074226 AddUsagePingFeaturesEnabledToApplicationSettings: reverted (0.0117s)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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