refactor: use credentials from tokenService

This refactoring reflects an insight that GitLab instanceUrl and personal access token are pair that exists in the app together.

So far we were passing the instanceUrl through as an argument and then later on, the logic called a global tokenService to get the token for the instanceUrl.

The problem is that the tokenService.getToken(instanceUrl) can return undefined but in many (if not all cases) we knew that the token exists.

This MR extends the usage of the Credentials interface, which is a pair of instanceURl and token.

Related to #515 (closed)