Add plan limits for Premium and Ultimate

What does this MR do?

Adds plan limits for Premium and Ultimate, copying from Silver and Gold, respectively. This is based on the investigation in #328783 (comment 619958871), where we found out these plan limits were missing.

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.

Database migration output

Added for compliance with the process, but it is not very meaningful because it copies at most records, and on my computer it does literally nothing. I suggest reviewing the tests thoroughly instead.

$ bin/rails db:migrate RAILS_ENV=test
== 20210706213537 AddPremiumAndUltimatePlanLimits: migrating ==================
== 20210706213537 AddPremiumAndUltimatePlanLimits: migrated (0.0000s) =========

$  bin/rails db:rollback RAILS_ENV=test
== 20210706213537 AddPremiumAndUltimatePlanLimits: reverting ==================
== 20210706213537 AddPremiumAndUltimatePlanLimits: reverted (0.0000s) =========