Store last_synced_at on licenses table

What does this MR do?

Related to #333871

As described in #333871, we want to show an accurate value for when the last sync occurred for a cloud license. This MR modifies the value used for last_sync in the CurrentLicenseType GraphQL type. For cloud licenses, this value will be pulled from a new datetime attribute last_synced_at introduced to the license object. For non-cloud licenses, the value will be nil. This MR also updates the activate service and the seat link sync service to set last_synced_at on new cloud license records.

Migration logs

Up

== 20210624180613 AddLastSyncedAtToLicenses: migrating ========================
-- add_column(:licenses, :last_synced_at, :datetime_with_timezone)
   -> 0.0039s
== 20210624180613 AddLastSyncedAtToLicenses: migrated (0.0040s) ===============

Down

== 20210624180613 AddLastSyncedAtToLicenses: reverting ========================
-- remove_column(:licenses, :last_synced_at, :datetime_with_timezone)
   -> 0.0092s
== 20210624180613 AddLastSyncedAtToLicenses: reverted (0.0114s) ===============

Screenshots (strongly suggested)

Cloud License example

Non-cloud license example

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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