[BUG] Channel custom metadata variable mapping disallows some common characters

[ChristopherSchultz]

When defining custom channel metadata, the "Variable Mapping" column appears to only accept letters and numbers. It's common to have other characters in map keys such as period . and hyphen -.

It would be nice to allow those characters, since the value itself is just a string and not e.g. a formal identifier in source code, etc.