Add setting to toggle Suggest Pipeline feature

What does this MR do and why?

As a first iteration to implement #337560 (closed) this MR adds suggest_pipeline_enabled column to application_settings table. This new setting defaults to enabled (current behavior) and will allow admins to toggle the Suggest Pipeline feature.

Suggest Pipeline MR widget

Migration output

db:migrate

== 20210921032008 AddSuggestPipelineEnabledToApplicationSettings: migrating ===
-- add_column(:application_settings, :suggest_pipeline_enabled, :boolean, {:default=>true, :null=>false})
   -> 0.0038s
== 20210921032008 AddSuggestPipelineEnabledToApplicationSettings: migrated (0.0038s)

db:rollback

== 20210921032008 AddSuggestPipelineEnabledToApplicationSettings: reverting ===
-- remove_column(:application_settings, :suggest_pipeline_enabled, :boolean, {:default=>true, :null=>false})
   -> 0.0032s
== 20210921032008 AddSuggestPipelineEnabledToApplicationSettings: reverted (0.0075s)

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