Disallow full stops in entry slugs

Full stops in slugs were preventing branch creation for unpublished
entries when using the editorial workflow, as `.` is not an allowed
character in branch names. This commit changes slug generation so
periods are replaced with `-` the same way other "non-sluggable"
characters are.

src/backends/backend.js

@@ -34,7 +34,7 @@ const slugFormatter = (template = "{{slug}}", entryData) => {
       case "day":
         return (`0${ date.getDate() }`).slice(-2);
       case "slug":
-        return identifier.trim().toLowerCase().replace(/[^a-z0-9\.\-_]+/gi, "-");
+        return identifier.trim().toLowerCase().replace(/[^a-z0-9\-_]+/gi, "-");
       default:
         return entryData.get(field, "").trim().toLowerCase().replace(/[^a-z0-9\.\-_]+/gi, "-");
     }