Add storage gaps into upgradeable contracts

[Mike-CZ]

This PR adds storage gaps into upgradeable contracts to avoid post-deployment upgradeability issues.

[thaarok]

What is the purpose of this?
From what I know, gaps are being added into parents/abstract contracts, to make some space to add more variables into the parent contract - to create gap before variables of the child:

000 variable of parent
001 variable of parent
002 gap in parent (new variable can be added into the parent here)
003 gap in parent
004 variable of child
005 gap in child (why???)

I don't see how is this going to help in the top-level contract...?

[thaarok]

Throughout the codebase, there are several contracts that are inherited by upgradeable contracts that do not include a storage gap

There are not. The mentioned contracts are top-level, they are never inherited by other contracts in production, only in few unit-test scenarios like UnitTestSFC/etc.
There is no reason for these gaps.

[thaarok]

As confirmed by auditors - since these contracts are NOT parents of other contracts, gaps or namespaced storage in these contracts will not bring any advantage.

[thaarok]

Already in prod, whatever...

The base branch was changed.