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/settings.py

@@ -275,10 +275,15 @@ class EnvSettings(LNbitsSettings):
     forwarded_allow_ips: str = Field(default="*")
     lnbits_title: str = Field(default="LNbits API")
     lnbits_path: str = Field(default=".")
+    lnbits_extensions_path: str = Field(default="lnbits")
     lnbits_commit: str = Field(default="unknown")
     super_user: str = Field(default="")
     version: str = Field(default="0.0.0")
 
+    @property
+    def has_default_extension_path(self) -> bool:
+        return self.lnbits_extensions_path == "lnbits"
+
 
 class SaaSSettings(LNbitsSettings):
     lnbits_saas_callback: Optional[str] = Field(default=None)