Unlocking 'Create Job' button when there's no job in progress

[ramongduraes]

A few suggested improvements on the dApp! I have unlocked the 'Create Job' button when:

1. The job contract hasn't been created yet, so that the user can choose another service if he has picked one by mistake, for example;
2. The job is complete, so that the user does not need to refresh the website every time a job is finished. In this case a button is rendered and the user is prompted to confirm that he wants to end the job (in which case the data is wiped).

Accomplished by sharing App's state variables as properties with Services and Job.