Force refresh when site is taken out of maintenance mode

[louise-davies]

Description:
This helps when doing a bugfix upgrade - we can ensure that no users have the old version of the code in their browser loaded after a certain period of time (i.e. how long SG checks for maintenance). For other circumstances - forcing a reload after maintenance mode is fairly non-intrusive - the only thing that is lost is any caching that the plugins may do. For this to work, we'd need to bump up the frequency of how often we check for maintenance mode changes - probably every 5 minutes is more suitable than every hour.

Acceptance criteria:

• When SG detects it's gone from maintenence mode true to false, it forces a page refresh. Easiest place to do this is probably the middleware I think. We don't need to use react-router reload method if it's more complex, basic window.location.reload() will do.
• Maintenance mode is checked for more frequently