feat(service): replace/refactor internal repository cache

[Panaetius]

Replaces the repository cache function contained in RenkuOperationMixin with a dedicated repository cache class.

closes #2242
closes #3493
closes #3482
closes #2797

/deploy #persist renku-gateway=master renku=core-svc-horizontal-scaling extra-values=core.sentry.dsn=https://0cc02f3a02d3474d9f7e90915873ab4d@sentry.dev.renku.ch/2,core.sentry.environment=renku-dev,core.sentry.enabled=true

[RenkuBot]

You can access the deployment of this PR at https://renku-ci-rp-3534.dev.renku.ch

[coveralls]

Pull Request Test Coverage Report for Build 5476285797

• 180 of 245 (73.47%) changed or added relevant lines in 21 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 85.702%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/ui/service/controllers/cache_migrations_check.py	8	9	88.89%
renku/ui/service/views/error_handlers.py	8	9	88.89%
renku/ui/service/cache/models/project.py	15	18	83.33%
renku/ui/service/gateways/gitlab_api_provider.py	12	15	80.0%
renku/ui/service/interfaces/repository_cache.py	10	13	76.92%
renku/ui/service/views/cache.py	11	14	78.57%
renku/ui/service/gateways/repository_cache.py	72	123	58.54%

Totals
Change from base Build 5476214874:	-0.2%
Covered Lines:	25768
Relevant Lines:	30067

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💛 - Coveralls

[olevski]

A few questions or minor (in most cases optional) changes.