List upcoming jobs

**Is your feature request related to a problem? Please describe.**
- To schedule new jobs for a new quality control, you might want to time it in such a way it doesn't coincide with a lot of other jobs.
- You might want an overview how many jobs you are running.
- Planning which failure notices you can skip in relation to planned maintenance/downtime.

**Describe the solution you'd like**
A simple API and/or overview page that list all upcoming jobs.

**Describe alternatives you've considered**
N.A.

**Additional context**
Just an idea for a feature enhancement. Currently no practical usecase other than curiosity about upcoming jobs. It is related to #247 but shows upcomming/planned runs rather than finished runs.

**Possible solution**
APScheduler can be configured to use a 'jobstore' (including SQLite) to store jobs (for `GHCrunner`). The `GHCwebapp` can access this information and show an overview page.