refactor: home page / url passing

We now allow multiple OPDS feeds to be pre-defined and displayed
on the homepage. As a result we need to pass the feed navigation
URLs via query parameter rather than a subpath which would only
support proxying to a single OPDS feed.

The feed is passed via the q= query parameter and any relative
links from the OPDS XML are resolved to a complete URL with
domain / scheme.

We also check the "Content-Type" header in the response
received from the OPDS feed to determine whether to parse
an OPDS catalog or just proxy the raw response back (images / files).

.vscode/launch.json

@@ -0,0 +1,16 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Launch Package",
+            "type": "go",
+            "request": "launch",
+            "mode": "auto",
+            "program": "${workspaceFolder}"
+        }
+
+    ]
+}