Updated contribution guidelines

We sadly still need to be more clear that tickets need to adhere to certain standards to be manageable...
Generate the key used for session hashing individually for each server instance
(cherry picked from commit 118a4f7)
Merge branch 'master' into devel

Conflicts:
	src/octoprint/server/__init__.py
	src/octoprint/settings.py
Allow login of the same user from multiple browsers without side effects

So far when logging in from two different browsers, then logging out in one of them the user was logged out across all browsers. This should now be changed in so far as that each individual browser session is tracked and only that session is ended by a logout that belongs to the browser where the logout button was clicked.

Should fix #556