add Scabot command for triggering community build

[SethTisue]

trafficstars

it's a bit silly we haven't automated this, but for a long time we didn't have enough capacity (mainly disk space) on our Jenkins instances

but now we do and it's silly to keep a human in the loop any more

[SethTisue]

the manual steps are:

• go to https://scala-ci.typesafe.com/view/scala-2.12.x/job/scala-2.12.x-integrate-community-build/
• log in to Jenkins (using your GitHub identity; we're happy to grant the needed access rights to anyone we regularly work with)
• hit "Build With Parameters" (if it doesn't appear, you forgot to log in or don't have sufficient access)
• and then for the version parameter, fill in e.g. 2.12.9-bin-abcd123-SNAPSHOT (where the version number is the next release, not the last release, and the 7-character SHA is from the last commit in the scala/scala PR in question)

same for 2.13.x, except (until 2.13.0 final is out) 2.13.0-pre- (instead of -bin-)

waiting for results can take many hours (as many as 15, and that's if your job is the only one in the queue, sometimes we have a couple days worth of runs stacked up)

you may want to check the run after a minute or two to make sure dependency extraction began, and then check it again after 20 minutes or so to make sure it proceeded from dependency extraction and into actually building projects

interpreting the results is sometimes easy and sometimes rather hard. often my knowhow is helpful on this, feel free to @-mention me for help

[dwijnand]

Be nice to add these instructions to the wiki too.

[NthPortal]

at the very least, we could give permission to the committers team to run the command if not everyone