GroupWikiPage: add slug property

Not needed for much yet, but should be used here anyway to prevent
"new_page" pages from being created inside folders (which isn't possible
yet anyway).

tildes/tildes/models/group/group_wiki_page.py

@@ -61,7 +61,7 @@ class GroupWikiPage(DatabaseModel):
         self.path = convert_to_url_slug(page_name)
 
         # prevent possible conflict with url for creating a new page
-        if self.path == "new_page":
+        if self.slug == "new_page":
             raise ValueError("Invalid page name")
 
         if self.file_path.exists():
@@ -99,6 +99,11 @@ class GroupWikiPage(DatabaseModel):
         """Return a relative path to the page's file."""
         return Path(str(self.group.path), f"{self.path}.md")
 
+    @property
+    def slug(self) -> str:
+        """Return the page's slug, which is also its filename with no extension."""
+        return self.file_path.stem
+
     @property
     def history_url(self) -> str:
         """Return a url to the page's edit history."""