webconfig themes: shell-friendly filenames

For historical reasons (namely the webconfig origin), our theme
names contain spaces and uppercase letters which can be inconvenient
when using the "fish_config theme choose" shell command.  Use more
conventional file names.

Web config still uses the pretty names, using the ubiquitous "# name:"
property.

share/tools/web_config/webconfig.py

@@ -1123,8 +1123,8 @@ class FishConfigHTTPRequestHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
             output = []
             output += [self.do_get_colors() | {"theme": "Current"}]
             output += [
-                self.do_get_colors("themes/fish default.theme")
-                | {"theme": "fish default"}
+                self.do_get_colors("themes/fish-default.theme")
+                | {"theme": "fish-default"}
             ]
 
             confighome = (