Add an automatic app-reload mechanism

[lunkwill42]

Problem

Currently, when argus-frontend is updated, users (typically, the NOC) must be told to manually reload their browsers in order to fetch and run the upgraded version of argus-frontend.

Suggestion

Could we build a mechanism for argus-frontend itself to detect that its source had been updated on the web server and reload itself?

Ideas for implementation

The mechanism itself could be something as simple as just retrieving a "flag file" from time to to time, and if this file had changed since argus-frontend first was loaded, it could initiate a browser page reload.

Other considerations

argus-frontend should only reload when "idle". I.e. it would be bad to reload argus-frontend while a user is actively engaging with. One thing is an "idle timer". If no user input had been received for a while, the user could be considered idle. However, if the currently active page is a notification form or an incident details dialog, the user could just be taking their time to decide what to input. It might be that we only wish to reload if the user has been idly watching the incidents list (which is the typical dashboard-in-NOC-screen-scenario).