frontend: cache the recent task queries
    
    So as part of this commit, initialize the flask_cache mechanism, using
    the already running redis database (we use it for session management).
    The 'cache' object logically belongs to __init__.py, though it needs to
    be placed on a pretty weird place (that's because there are
    bi-directional dependencies among our modules, this is a problem but to
    be solved in a different PR).
    
    Also, instead of caching get_recent_tasks() directly, I created
    get_recent_task_ids() which is safe for caching.  That's because the
    contents is just a list of integers, not sqlalchemy objects.
    
    The problem of caching sqlalchemy objects is that they allow
    lazy-loading later in templates, and if loaded from cache - when the
    object isn't bound to any session - the lazy load attempt would fail.
    Therefore we cache just the IDs, and before passing the objects down to
    the template we perform another (but way cheaper) db query.