BB2-3532: Added check for flag for ag_expiration info

[loganbertram]

JIRA Ticket:
BB2-3532

What Does This PR Do?

Adds a check for whether or not the feature flag for data access grants is active. If it is not, then the ag expiration date will always be an empty string.

What Should Reviewers Watch For?

While testing, use your local app and uncheck the "Superusers" box on the waffle flag for "limit_data_access". This option will otherwise pretend that the flag is always on for superuser accounts, complicating testing.

If you're reviewing this PR, please check for these things in particular:

Validation

Create a local app, set the local waffle flag superuser option to false, then get an access token for your app. The expiration date field should be an empty string. If you flip the flag back on for superusers or enable it for your app, you will see a date again.

What Security Implications Does This PR Have?

Please indicate if this PR does any of the following:

• Adds any new software dependencies
• Modifies any security controls
• Adds new transmission or storage of data
• Any other changes that could possibly affect security?

• Yes, one or more of the above security implications apply. This PR must not be merged without the ISSO or team
  security engineer's approval.

Any Migrations?

• Yes, there are migrations
  • The migrations should be run PRIOR to the code being deployed
  • The migrations should be run AFTER the code is deployed
  • There is a more complicated migration plan (downtime,
    etc)
• No migrations

[JFU-NAVA-PBC]

LGTM

Tested on local bb2 server and verified the expected DAG expiration value.

Approved - assuming there is no need to retrofit existing DAG records