Add models for dependency proxy ttl policies

🔎 What does this MR do?

The dependency proxy is a pull-through cache for DockerHub images. Currently, users can clear the entire cache, but there are no automated controls over this functionality. In #294187 (closed) we are adding TTL (time-to-live) policies so users can adjust to automatically clear cached files older than a specific number of days (90 by default).

In this MR, we add the initial table and model that will be used to manage these policies.

💾 Database

Migration Output:

→ be rake db:migrate:redo VERSION=20210823213417
== 20210823213417 CreateDependencyProxyImageTtlGroupPolicies: reverting =======
-- drop_table(:dependency_proxy_image_ttl_group_policies)
   -> 0.0035s
== 20210823213417 CreateDependencyProxyImageTtlGroupPolicies: reverted (0.0294s)

== 20210823213417 CreateDependencyProxyImageTtlGroupPolicies: migrating =======
-- create_table(:dependency_proxy_image_ttl_group_policies, {:id=>false, :primary_key=>:group_id})
   -> 0.0127s
== 20210823213417 CreateDependencyProxyImageTtlGroupPolicies: migrated (0.0201s)

Screenshots or Screencasts (strongly suggested)

N/A

How to setup and validate locally (strongly suggested)

N/A

✏ Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
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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

Related to #294187 (closed)