clean_refs plugin cleans refs overly eagerly

[kfogel]

On the jinjification branch, as of commit 801b4c3, the pipeline/plugins/60_clean_refs.py plugin is perhaps too eager about cleaning out refs. We have a document containing this line in the LaTeX source:

TODO: address ref:bf388517

which right now of course gets rendered as:

TODO: address

We will sometimes want to be able to name refs in documents so that they're visible. One solution would be to tighten the regexp, so that it only cleans refs that are surrounded by parentheses or by square braces. Another solution would be to take ref-cleaning out of that stage of the pipeline entirely and instead just create a standard \ign{ref:blahblah} LaTeX command that expands its argument to the empty string. Actually we already have this, with \otsref as the command instead of \ign, but maybe the latter would be better because it's shorter.

Filing this issue so we don't forget about the problem, though it's certainly not an urgent thing. I've marked the issue with the "help wanted" label because I'm not positive we should declare this a bug, though I do lean that way. Anyway, one can always work around it right now by doing something like this:

TODO: address ref:\otsref{fish}bf388517