Validate ignore regexes before committing them to jobs

[hannahwhy]

[14:04:28] <DFJustin> the !ig command should probably validate the regex before adding it
[14:04:34] <xmc> DFJustin: good point
[14:04:43] <yipdw> it should, but the bot's in Ruby and the pipeline's in python
[14:05:05] <yipdw> that said it wouldn't be difficult to shell out to a Python process and go "does this work y/n"

[hannahwhy]

There's also the possibility that Ruby and Python regexes are close enough that shelling out wouldn't be needed.