Calendar improvement: C-j now just opens an entry in another window without selecting it

One more minor improvement. :-)

Consider the following case. Sometimes I just want to browse my journal entries in the calendar without any edits. Pressing "j" will inevitably open a new window, select it... To get back to the calendar I have to press an extra button. In dired you can check out files without selecting the buffer (o vs C-o vs C-M-o). So I added one more calendar-mode-map keybinding, which does exactly that and - hopefully - is easy to remember: C-j. 

I refactored out some code into a common function - org-journal-read-or-display-entry, which chooses between displaying and switching.