Always expand all paths (journals, templates, etc)

[apainintheneck]

This also fixed bugs with relative journal and template paths.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

This is related to the discussion in issue #998. I took a look at the path expansion code used throughout the project and decided to create a new path.py file to hold the three common cases.

1. Getting the path to the home directory.
2. Expanding shell variables and the user path (~).
3. Getting the absolute path after expanding shell variables and the user path (~). Note: This used to happen in the opposite order which is probably what caused problems.

Additionally, a call to expand_path is now made before loading template files which solves the problem with loading files like ~/template. I'm still not sure though if the error message after failing to load a template should print the original template path string or the expanded one (line 210 in jrnl/jrnl.py).

Closes #998.

[wren]

This looks great. Just one comment about the new functions to fix.

Also, would you mind adding a test or two? I think just a couple of unit tests for the new functions should be more than enough.

[wren]

I think we could do with just home_dir and expand_path as long as expand_path also runs abspath. Would you mind merging expand_path and absolute_path into one function?

[apainintheneck]

I tried that at first because I assumed that it wouldn't make a difference but I think that's not a great idea for a couple reasons.

1. It breaks the tests as currently written. A few BDD tests check the console output of the application and the output that gets parsed contains relative paths. That means that those tests would have to changed to parse the absolute path which is a bit more complicated.
2. The absolute path is only needed once so it would add extra work to all the other calls.
3. Console error messages are more readable when they display the relative path instead of the absolute path.

[apainintheneck]

I'm currently running into some problems creating tests that work for both platforms. Essentially, the call to os.path.abspath() internally calls os.getcwd(). My first thought was to just mock that in pytest but that didn't work on Windows for some reason. Then, I decided to use os.getcwd() directly and got FileNotFoundErrors on *nix.

[wren]

I went ahead and updated the tests to fix the issues you were running into.

The rest looks good. Thank you!