Convert the users store to use the basestore package

Created by: asdine

• Rename the users store to userStore https://github.com/sourcegraph/sourcegraph/blob/fb44dc1feefe43ffadf98c47c7f1d450cefee844/internal/db/users.go#L29-L29

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