add option to store markdown backups elsewhere

`--md-hist` / volflag `md_hist` specifies where to put old
versions of markdown files when edited using the web-ui;

* `s` = create `.hist` subfolder next to the markdown file
   (the default, both previously and now)

* `v` = use the volume's hist-path, either according to
   `--hist` or the `hist` volflag. NOTE: old versions
   will not be retrievable through the web-ui

* `n` = nope / disabled; overwrite without backup

tests/util.py

@@ -174,6 +174,7 @@ class Cfg(Namespace):
             lang="eng",
             log_badpwd=1,
             logout=573,
+            md_hist="s",
             mte={"a": True},
             mth={},
             mtp=[],