Move web interface to configure application into src/interface/web directory

- Improve code layout by ensuring all web interface specific code
  under the src/interface/web directory
- Rename config API to more specifi /config instead of /ui
- Rename config data GET, POST api to /config/data instead of /config

src/interface/web/index.html

@@ -85,7 +85,7 @@
         function populate_type_dropdown() {
             // Populate type dropdown field with enabled search types only
             var possible_search_types = ["org", "markdown", "ledger", "music", "image"];
-            fetch("/config")
+            fetch("/config/data")
                 .then(response => response.json())
                 .then(data => {
                     document.getElementById("type").innerHTML =