Add granular token enforcement to application settings

What does this MR do and why?

• Adds personal_access_token_settings JSONB column to application settings.
• Inside that we add two settings - enforce_granular_tokens (boolean) & granular_tokens_enforced_after (date).
• If feature flag, granular_personal_access_tokens_enforcement is enabled, admins set a date after which granular tokens will be enforced.
• If an admin sets a date, creating a legacy personal access token will be disabled for all users on the instance. Will add the enforcement in a follow-up as part of #579727

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag:

Feature.enable(:granular_personal_access_tokens_enforcement)

2. Log-in as an admin and navigate to admin/application_settings/general and check the box for & set a date for the field, Require fine-grained personal access tokens after a specific date.
3. Confirm the correct date is set in the console:

Gitlab::CurrentSettings.enforce_granular_tokens
Gitlab::CurrentSettings.granular_tokens_enforced_after

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Related to #579726