Handle unsafe windows filenames properly

[Gum-Joe]

Currently, when you do a mailmerge you get asked what fields from the data source should be used to generate the output filenames.

Unfortunately, sometimes those fields contain invalid characters (e.g. ":", can't include that in a filename in Windows). This can cause mailmerge generation to silently fail, and files end up created with nothing written to them (very easy to miss).

Solution: Make sure to normalise any fields used for filenames before using them as a filepath. Need to also make sure that any references made to those files in e.g. JSONSidecars or nunjucks metadata are also updated.