Add description field to cluster agent token

What does this MR do?

We're building a new token display, which includes a description field. See issue: #298748 (closed)

The MR will

• Add description field to Clusters::AgentToken
• Update GraphQL and Clusters::AgentTokens::CreateService to allow users to enter a description field
• Update associated tests and translations

Screenshots (strongly suggested)

description

Mutations

up

== 20210216193620 AddDescriptionToClusterToken: migrating =====================
-- column_exists?(:cluster_agent_tokens, :description)
   -> 0.0016s
-- add_column(:cluster_agent_tokens, :description, :text)
   -> 0.0032s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0001s

-- execute("ALTER TABLE cluster_agent_tokens\nADD CONSTRAINT check_4e4ec5070a\nCHECK ( char_length(description) <= 1024 )\nNOT VALID;\n")
   -> 0.0035s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- execute("ALTER TABLE cluster_agent_tokens VALIDATE CONSTRAINT check_4e4ec5070a;")
   -> 0.0012s
-- execute("RESET ALL")
   -> 0.0006s
== 20210216193620 AddDescriptionToClusterToken: migrated (0.0224s) ============

down

== 20210216193620 AddDescriptionToClusterToken: reverting =====================
-- remove_column(:cluster_agent_tokens, :description)
   -> 0.0108s
== 20210216193620 AddDescriptionToClusterToken: reverted (0.0109s) ============

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

Related to #300417 (closed)