Always expand all paths (journals, templates, etc) (#1468)

* Refactored path expansion calls into a new path.py file

This also fixed bugs with relative journal and template paths.

* Update tests for new path functions

Also, make the tests specific to Windows, Mac & Linux

Co-authored-by: Jonathan Wren <jonathan@nowandwren.com>
Co-authored-by: Micah Jerome Ellison <micah.jerome.ellison@gmail.com>

jrnl/path.py

@@ -0,0 +1,13 @@
+import os.path
+
+
+def home_dir():
+    return os.path.expanduser("~")
+
+
+def expand_path(path):
+    return os.path.expanduser(os.path.expandvars(path))
+
+
+def absolute_path(path):
+    return os.path.abspath(expand_path(path))