Arrange for ALL/ALL.xml link or file

The cobra XML file is saved in pfs_instdata/data/pfi/modules/ALL/, currently with a manually chosen version suffix on it ("ALL_final_20210920_mm.xml"). To allow sane butler access, we should choose the filename format, and always have a single "current" version, i.e. ALL/ALL.xml.

I propose that individual versions be checked in with a common datestamp format, and that we update and check in a link to the current best version. "diff"s of XML files are rarely useful, so I don't think it is useful to have a single file with history.

So, save ALL/ALL_%Y-%m-%d_

.xml and an ALL/ALL.xml link to that.

For right now, I'll add and push a link without doing anything else.