GitLab CE features to work with unlicensed EE instance

# Background As part of making [EE the default download](https://gitlab.com/gitlab-org/gitlab-ee/issues/2417) on GitLab, we need to allow a GitLab EE instance to function as a free CE instance when it doesn't have a license. # Proposal * All GitLab CE features to work on GitLab EE when no license is found * No GitLab EE features should work when there isn't a regular or trial license * Adding a trial license should activate GitLab EE features for a limited number of days (30) * Once the trial license expires, GitLab CE features should continue to work * Once a trial license expires, there should be a notification and link to contact sales to purchase a full EE license * If a regular license expires, the server should no longer function and require a renewal (this is current EE behaviour) # Out of scope * There is a separate issue to generate a trial license inside the product or to purchase a full license https://gitlab.com/gitlab-org/gitlab-ee/issues/2501 * There will be separate issues to promote GitLab EE features when no license is present # Questions for consideration * If a trial license expires, what is the behaviour for regular users vs admins? # Design ### Admin | Trial license will expire | Trial license expires | | ------------------------- | --------------------- | |  |  | ### Regular user  - When the trial license remains 7 days, there is a message to remind users of this above the header. - If the trial license expires, there will be a message to show the status of trial license expires above the header. - Once users click `X`, the message will disappear. # Related issues * [META Issue](https://gitlab.com/gitlab-org/gitlab-ee/issues/2417) * [GitLab.com subscription license checks](https://gitlab.com/gitlab-org/gitlab-ee/issues/2335)

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