Re-implement building binaries via compile farm

[andythenorth]

Current

Alongside the move to GitHub, the compile farm for building binaries (nightly, stable etc) is being re-implemented. At time of writing (September 2018) the old compile farm infrastructure is not being used, and the new infrastructure is not implemented yet.

TrueBrain made a Trello list of what needs to be done for new infra. https://trello.com/b/6j90aRB1/openttd

Desired

New infra is implemented and in production.

[TrueBrain]

A lot of work is done, and can been seen here:

https://github.com/OpenTTD/OpenTTD-CF

[TrueBrain]

Reminder: we decided to only produce 64bit OSX binaries. See #6978 for more detail.

[TrueBrain]

Currently in testing:
https://www.staging.openttd.org/

If all checks are completed, it will be moved to production (read: https://www.openttd.org)

[andythenorth]

@TrueBrain want the glory of closing this one? :)