Add buttons for subscription management

What does this MR do?

This MR is part of the work done to address gitlab-org/gitlab#327248. In this context, we're exposing the Customers Portal URL to the Vue app in order to later retrieve it and use it for the Manage button.

Expose Customers Portal URL to Vue app	!61338 (merged)
Subscription Details Buttons	👈 you are here
License Remove Button and Modal	tbd

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.

To test the changes locally

1. Delete any previous license you might have

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. To test a Legacy License (you should not have any another license), use this key:

eyJkYXRhIjoiREJGb0g2SzRpWTZwY2lBVXFyWTFkTUNhb3dZTS9hbWhCd0l2
ZEdTMnZTd3ZUazZvR2YvK3pNVzJWMW1RXG5Vcm9DU0lGeFlHOExBNC9hSzZ4
VVk1eEZCUkhhcmF0b09BK0lKWGZkaDdnbERwc2xwVkV6MGRvOVBSR3ZcbkVh
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ZzBIU2lIZFlMRCsyeVxuZmJtcG5KNXRsY29TV0RhNTJGN0NCdTU0NGNMa1Nt
WjZnYi9VYkFoUExOMkZDTGR1REsxUGNxcXRCWThWXG5rWS9Kc0hFWDBMSFlV
OVVncTE4ZnJmRUt1UGw0TnoyVkdJV1BXOVJ1Y00wamhRWW1WSUczL1dqOUpN
VnZcbkR2bEdybmpybmdxZzVaUEdPL0V5SXAvL2sxVmtnQ3p5NnVFMTJnc3hx
M0pMWUNrZ01mMEJoNTlxSS9mNVxuSVRvVVlnbHUyRWRqczk0ZkRYaEFxeGdm
aUdwWng4MnNtUFNPVVFMUnFqMXZpeStLd00wN3dHSEJ4R2pCXG5oRDZSTC8y
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ImtleSI6IlVyemhZZU9mWTB1MzR0aGE5Tm1uSmdpb1BkeFlsK3lCbTNYajhh
WDV6elU0bzd4RHFJQWhIU3NReUh5Q1xuVlYzQ1lEYmtjZGd5T2hZdVNKUDNN
cjJsVi83ZHpjN1VQRTd0NFhIVmVVcXZtTm9DcVZycDhHMkZ5YkltXG5YQmRP
UnZMVVNVdkJ2Unc5dlhudTA4WXZ4Z0JqYTBjcEgrTGRvTS9ING5XSzJUSk9w
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a3lrOXpiQlpQUnBYODRXZEVRVEQ3eWcvb0c4SjloVlQzZTlVYmRHZytuYmpU
XG5GWW9Zd2l4aTlaNGFPVHZmc1lSeWpONkxDNkdRUEpLaHRrQjU1ODdkcXc9
PVxuIiwiaXYiOiJCWXNSTjQ5alRsVTR2bVYzUEJDZGpBPT1cbiJ9

5. To test a Cloud License, use this code: V6SCyxa5Gj1wmNVY65PnbBsc

Screenshots (strongly suggested)

Loading	Cloud Type	Legacy Type

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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