Draft: Update license to be undecryptable

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/324013

When a sync seat request failed due to an undecryptable license, update the license accordingly. A new attribute called decryptable was introduced for this to the License model.

This information will be used later to display a banner in the UI.

Migration

up:

== 20210520111905 AddDecryptableToLicenses: migrating =========================
-- add_column(:licenses, :decryptable, :boolean, {:default=>true})
   -> 0.0057s
== 20210520111905 AddDecryptableToLicenses: migrated (0.0058s) ================

down:

== 20210520111905 AddDecryptableToLicenses: reverting =========================
-- remove_column(:licenses, :decryptable, :boolean, {:default=>true})
   -> 0.0028s
== 20210520111905 AddDecryptableToLicenses: reverted (0.0069s) ================

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