User.last_login stores the last time that a user went through a login

process, but given how long our sessions last, it's hard to get any

useful information on how often users are active on a server. This

change aims to improve upon this by repurposing this field to better

show recent activity.
Now, on each request, the new UpdateLastLoginMiddleware will check if

the last login time is at least 30 minutes old and update it to the

current time. This means that login times will now represent activity on

the server within a 30 minute window, making the User Activity graph on

the Admin Dashboard more useful and helping when calculating usage for

support contracts.

Unit tests passed.
Manually tested that the last login time was updated when making a HTTP

request, and then didn't update again until a request over 30 minutes

later.