Allow to queue run of manual stage while other stage is running

Example:

1. Have a build plan with two stages, e.g. "Default Stage" and a manual "Func Tests" stage.
2. Start a build, so that "Default Stage" is running.

Expected: I can queue running the "Func Tests" stage while the first stage is still running.

Actual: The UI makes me wait until the first stage is finished, before the button becomes enabled and I can click it. That requires manually checking the status and actually being online to start the other stage.