Reduce duplicated code and expose a function for getting the enonic home dir

[eiriksm]

This fixes #570

So if I wanted to make sure the directory was possible to override, I would first consolidate all different places the path to $HOME/.enonic was found.

Here is a first attempt to do that. Plus some actual tests for this repo

[pmi]

Hi @eiriksm !

Thank you for the effort !

Would you be interested to extend your PR with actual override mechanic as you see it ?

NB. Overriding the default folder might get a bit tricky in case of running in snapcraft where we create a symlink to .enonic folder because of their execution policy.

[eiriksm]

Sure

Here is the updated logic, including a test

[eiriksm]

No idea how to build the docs, but attempted to add it there. Updated the var name, and made it a const

[pmi]

Thanks ! Looks good ! Docs are built by github in master so we'll wait with merging until release !