Convert the userExternalAccounts store to use the basestore package

Created by: asdine

• Rename the userExternalAccounts store to userExternalAccountStore https://github.com/sourcegraph/sourcegraph/blob/59f929c20a81b69b2ebdc4c583ebd092e7afc091/internal/db/external_accounts.go#L33-L33

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