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>

.env.example

@@ -40,6 +40,11 @@ LNBITS_HIDE_API=false
 # GitHub has rate-limits for its APIs. The limit can be increased specifying a GITHUB_TOKEN
 # LNBITS_EXT_GITHUB_TOKEN=github_pat_xxxxxxxxxxxxxxxxxx
 
+# Path where extensions will be installed (defaults to `./lnbits/`).
+# Inside this directory the `extensions` and `upgrades` sub-directories will be created.
+# LNBITS_EXTENSIONS_PATH="/path/to/some/dir"
+
+
 # Extensions to be installed by default. If an extension from this list is uninstalled then it will be re-installed on the next restart.
 # The extension must be removed from this list in order to not be re-installed.
 LNBITS_EXTENSIONS_DEFAULT_INSTALL="tpos"