Added keybinds to open a subreddit, open a user page, and for leaving or joining a subreddit

Added keybinds to open subreddit and user page, conform merge request 688 from goggle on the original source. (see https://github.com/michael-lazar/rtv/pull/688), and updated the code to fix a potential crash. Additionally added shortcuts to leave or join while browsing a subreddit.

All shortcuts that are added are:

• 'v' view subreddit of selected item
• 'V' view user page of selected item
• 't' join the current subreddit
• 'T' leave the current subreddit