Let --sessionfile default to a more persistent path (#659)

The presence of a sessionfile avoids the usage of the failure-prone login
mechanism.  This commit addresses a suggestion in #615 to store the sessionfile
in a persistent path rather than a path within a temporary directory if no
--sessionfile parameter is given.

The default path is now:

  $XDG_CONFIG_HOME/instaloader/session-USERNAME or
    ~/.config/instaloader/session-USERNAME on Unix,

  %LOCALAPPDATA%\Instaloader\session-USERNAME on Windows.

If no file exists in the new path, Instaloader tries loading from the path
where the sessionfile was stored before this commit, hence it automatically
migrates to the new sessionfile path.

instaloader/instaloader.py

@@ -27,6 +27,20 @@ from .structures import (Hashtag, Highlight, JsonExportable, Post, PostLocation,
 
 def get_default_session_filename(username: str) -> str:
     """Returns default session filename for given username."""
+    sessionfilename = "session-{}".format(username)
+    if platform.system() == "Windows":
+        # on Windows, use %LOCALAPPDATA%\Instaloader\session-USERNAME
+        localappdata = os.getenv("LOCALAPPDATA")
+        if localappdata is not None:
+            return os.path.join(localappdata, "Instaloader", sessionfilename)
+        # legacy fallback - store in temp dir if %LOCALAPPDATA% is not set
+        return os.path.join(tempfile.gettempdir(), ".instaloader-" + getpass.getuser(), sessionfilename)
+    # on Unix, use ~/.config/instaloader/session-USERNAME
+    return os.path.join(os.getenv("XDG_CONFIG_HOME", os.path.expanduser("~/.config")), "instaloader", sessionfilename)
+
+
+def get_legacy_session_filename(username: str) -> str:
+    """Returns legacy (until v4.4.3) default session filename for given username."""
     dirname = tempfile.gettempdir() + "/" + ".instaloader-" + getpass.getuser()
     filename = dirname + "/" + "session-" + username
     return filename.lower()
@@ -441,6 +455,8 @@ class Instaloader:
         """
         if filename is None:
             filename = get_default_session_filename(username)
+            if not os.path.exists(filename):
+                filename = get_legacy_session_filename(username)
         with open(filename, 'rb') as sessionfile:
             self.context.load_session_from_file(username, sessionfile)
             self.context.log("Loaded session from %s." % filename)