Move agelimit and newlimit into settings

[jace]

The agelimit and newlimit variables should be in settings. However, importing timedelta in the settings file is a no-no (no code in settings), so we do it like this:

# settings.py
AGE_LIMIT = {'days': 30}
NEW_LIMIT = {'days': 1}

# code
agelimit = timedelta(**app.config['AGE_LIMIT'])
newlimit = timedelta(**app.config['NEW_LIMIT'])

[jace]

Since the 30 day age limit has now become a defining product feature, with hardcoded references to "30 days" in several places, I propose we forget about this. There is no pressing need to move it into settings.