Allow project slugs to be reused after project deletion

Carryover proposal item (5) from the MVC released in #24866 (closed)

Problem to solve

In the past this proposal was not easily possible as the repositories were named the same as their namespace/project_slug. With Hashed Storage, this is no longer an issue and we can safely delay the removal while still allowing a new project to be created with the exact same name as the previous one.

Proposal

TBD

Documentation