Show last update / refresh time for Dashboard statistics

### Prerequisites

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- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Problem Description

I know that hovering over one of the graphs will show the number of requests and time for each hour, but it would be nice if it was immediately obvious what time period the stats are for. I keep AGH open in a pinned tab so I can easily check it whenever, so knowing the time period shown before and after refreshing the data would be good for keeping an eye on unexpected network traffic.

### Proposed Solution

Either show the date and time of when the Dashboard page loaded or the period covered by the current set of stats, as in the two screenshots below.