add rw_edit (closes #1276);

previously, only .md files were editable with permissions read+write;
all other file-extensions required the delete-permission in addition

rw_edit is the list of file-extensions to allow read+write editing of

tests/util.py

@@ -212,6 +212,7 @@ class Cfg(Namespace):
             rm_retry="0/0",
             rotf_tz="UTC",
             rss_sort="m",
+            rw_edit="md",
             s_rd_sz=256 * 1024,
             s_wr_sz=256 * 1024,
             shr_who="auth",