Small usability fixes

Typically small fixes, that improve user experience.

• Set _R_CHECK_FORCE_SUGGESTS_ to false on Windows, by default.

• Allow overriding R_CHECK_FORCE_SUGGESTS_ on Linux. (Previously it was set after the user supplied environment variables.)

• Use our CRAN mirror on Windows as well. For this I had to make it public, because the windows builders are classic Azure machines, so they are on a different internal network. I also installed HTTPS on it, via letsencrypt.

• Fixed the Jenkins job reaper, the rhub-cron process that removes old jobs from Jenkins.

• Updated the R-hub API, so that a job can be submitted for multiple platforms. This is needed for the check_for_cran() function in rhub.

• Added the check_for_cran() function to the rhub package. It runs the checks on 3-4 platforms, depending on whether the package has compiled code.