(feat) add repo meta write rbac

[erzhtor]

Part of https://github.com/sourcegraph/pr-faqs/issues/96.

This PR:

• adds REPO_METADATA#WRITE RBAC
• adds RBAC check for repo metadata create/delete/update
• adds RBAC check for UI repo metadata edit page

Test plan

• sg start
• Enable repository-metadata feature flag
• Go to any repository page and check that there is a gear icon/link available near metadata. Click it and it should open repo metadata edit page
• Uncheck REPO_METADATA#WRITE permission for your current non-admin user https://sourcegraph.test:3443/site-admin/roles. NOTE: that admin users will have all permissions and they are not revokable
• Go to any repository page and check that there is NO gear icon/link available near metadata

Screenshots

Screen.Recording.2023-05-17.at.21.28.33.mov

[sourcegraph-buildkite]

Bundle size report 📦

Initial size	Total size	Async size	Modules
0.01% (+0.30 kb)	0.02% (+2.25 kb)	0.02% (+1.96 kb)	0.00% (0)

Look at the Statoscope report for a full comparison between the commits f30052f and 97910a6 or learn more.

Open explanation

• Initial size is the size of the initial bundle (the one that is loaded when you open the page)
• Total size is the size of the initial bundle + all the async loaded chunks
• Async size is the size of all the async loaded chunks
• Modules is the number of modules in the initial bundle

[sourcegraph-bot]

📖 Storybook live preview