Add application rate limit for Notes creation

What does this MR do?

Related to #320792

Adds a rate limit for the creation of Notes via web, API and GraphQL endpoint. The limit has a default value of 300 and can be modified in Admin Area > Settings > Network > Notes Rate Limits.

GraphQL request example

mutation {
  createNote(input: { body: "test", noteableId: "gid://gitlab/Issue/2752" }) {
    errors
  }
}

GraphQL request response example

{
  "data": {
    "createNote": null
  },
  "errors": [
    {
      "message": "This endpoint has been requested too many times. Try again later.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "createNote"
      ]
    }
  ]
}

Migration 20210208161207_add_notes_create_limit_to_application_settings

rails db:migrate:down VERSION=20210208161207

== 20210208161207 AddNotesCreateLimitToApplicationSettings: reverting =========
-- remove_column(:application_settings, :notes_create_limit, :integer, {:default=>300, :null=>false})
   -> 0.0014s
== 20210208161207 AddNotesCreateLimitToApplicationSettings: reverted (0.0026s)

rails db:migrate:up VERSION=20210208161207

== 20210208161207 AddNotesCreateLimitToApplicationSettings: migrating =========
-- add_column(:application_settings, :notes_create_limit, :integer, {:default=>300, :null=>false})
   -> 0.0025s
== 20210208161207 AddNotesCreateLimitToApplicationSettings: migrated (0.0025s) 

Screenshots (strongly suggested)

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

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