Convert the userCredentials store to use the basestore package

Created by: asdine

• Rename the userCredentials store to userCredentialStore https://github.com/sourcegraph/sourcegraph/blob/e74d5acb98eeda0ed7ae8738056c1dc398e32bcc/internal/db/user_credentials.go#L49-L49

• Convert the store to use the basestore package so that it can be instantiated without using the global db connection https://github.com/sourcegraph/sourcegraph/blob/a547a87f9d3a447be90e2c53760f4792799e8e2c/internal/db/basestore/store.go#L10-L36

Example: https://github.com/sourcegraph/sourcegraph/blob/main/internal/db/repos.go#L55-L73

• Make sure the global userCredentials store is usable without initializing it https://github.com/sourcegraph/sourcegraph/blob/e612c34f2e27005928ff3dfdd8e5ead5a0cb1526/internal/db/repos.go#L75-L85