Add application limits to instance level CI/CD variables

What does this MR do?

Related to #216097 (closed)

Adds a hard limit on how many instance level CI/CD variables can be defined.

bin/rake db:migrate

== 20200519141534 AddInstanceLevelVariablesColumnToPlanLimits: migrating ======
-- add_column(:plan_limits, :ci_instance_level_variables, :integer, {:default=>25, :null=>false})
   -> 0.0041s
== 20200519141534 AddInstanceLevelVariablesColumnToPlanLimits: migrated (0.0042s)

bin/rake db:rollback

== 20200519141534 AddInstanceLevelVariablesColumnToPlanLimits: reverting ======
-- remove_column(:plan_limits, :ci_instance_level_variables, :integer, {:default=>25, :null=>false})
   -> 0.0014s
== 20200519141534 AddInstanceLevelVariablesColumnToPlanLimits: reverted (0.0028s)

Screenshots

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

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• 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