Add MVP authentication to backend

Hard-coded for now
Auth fixes
Show failed login error
Style login page
Restructure backend views to remove navbar from un-logged in pages
When unauthorized, only add redirect parameter when the request isn't just /admin
Add logout functionality
Move some auth code from route to controller
Give `backend_post_edit` view access to query parameters
Add todo comments
Specify default express-session options to avoid console error
Custom cookie name...just 'cause
Use environment vars for username, password and express session secret
Move auth middleware to method of backend controller
Add dropdown to view post types in backend list
Tidy up auth methods