Ability to download and upload config files (#1083)

- Rename `/config/load` endpoint to `/config/action` since it has multiple functions
- Add a `send` function to above endpoint, which triggers a download of the config file, and use it with a new Download button on the index page
- Add an option to upload to the CFG_DIR
- Improve layout of the file transfer destination web form: radio buttons before labels, and better padding between options
- Add a test for config downloading

python/web/src/static/themes/modern/style.css

@@ -49,6 +49,10 @@ a:hover {
   text-decoration: none;
 }
 
+input[type="radio"] {
+  margin: 0 0.1rem 0 0.75rem;
+}
+
 /*
  ------------------------------------------------------------------------------
  Tables
@@ -145,10 +149,6 @@ select {
   align-items: center;
 }
 
-.dropzone label {
-  padding: 0 0.5rem;
-}
-
 .dropzone p,
 .dropzone .dz-default {
   flex: 0 0 100%;