Support custom path for installed extensions (#1940)

* feat: the more you fuck around the more you learn
* feat: backwards compatible static file loading
* refactor: update paths for extension files
* refactor: var renaming
* doc: update `LNBITS_EXTENSIONS_PATH` documentation
* fix: default folder install
* feat: install ext without external path
* doc: `PYTHONPATH` no longer required
* fix: add warnings
* fix: missing path
* refactor: re-order statements
* fix: hardcoded path separator

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Co-authored-by: dni ⚡ <office@dnilabs.com>

lnbits/helpers.py

@@ -30,6 +30,10 @@ def url_for(endpoint: str, external: Optional[bool] = False, **params: Any) -> s
 def template_renderer(additional_folders: Optional[List] = None) -> Jinja2Templates:
     folders = ["lnbits/templates", "lnbits/core/templates"]
     if additional_folders:
+        additional_folders += [
+            Path(settings.lnbits_extensions_path, "extensions", f)
+            for f in additional_folders
+        ]
         folders.extend(additional_folders)
     t = Jinja2Templates(loader=jinja2.FileSystemLoader(folders))