Add button for mid-term upgrade

What does this MR do?

Relates to gitlab-org/gitlab#332490, this MR is:

• displaying a new button when there's an active subscription, with type file, that triggers a modal
• adjusting the condition to display the previous enter activation code button

A feature flag is not used because this feature is for self-managed instances. The feature is behind an Application Setting and it's not released nor documented. To test it locally, see the following steps.

Local Testing

To see the button

1. You should have a (legacy) license in your instance. You can refer to this for how to do that.

2. Make sure you have set the following environment vars (usually in your terminal) and restart gdk

   • GITLAB_LICENSE_MODE is set to test
   • CUSTOMER_PORTAL_URL is set to https://customers.stg.gitlab.com

3. Use Rails console to run ApplicationSetting.current.update(cloud_license_enabled: true)

4. Visit Admin > Subscription

Screenshots (strongly suggested)

Activate cloud license button	Enter activation code button	Modal
		Screen_Recording_2021-06-17_at_17.59.55

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?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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