Display last time merge bot queue was polled in home page

When you go to `mergit.build-leapyear.com`, you should be able to see the last time the merge bot polled the queues in each repo.

General areas to fix:
* Add a new value to `BaseAppConfig` of type `IORef UTCTime`, which will contain the last time the merge bot polled the queues ([`Mergit.Monad`](https://github.com/LeapYear/mergit/blob/f413b8be4ae50f624020c90eecce585efe069de0/mergit/src/Mergit/Monad.hs#L80-L84))
* Edit [`pollMergeQueues`](https://github.com/LeapYear/mergit/blob/f413b8be4ae50f624020c90eecce585efe069de0/mergit/src/Mergit.hs#L133-L139) to update the IORef with the current time, after polling the queues in all registered repos
* Read the IORef and edit the HTML in `Mergit.Routes.Debug`

Start in mergit/src/Mergit/Monad.hs at BaseAppConfig and in mergit/src/Mergit.hs at pollMergeQueues. Then read Mergit.Routes.Debug to understand the home-page HTML and how the timestamp should be displayed. Done means the page shows when the merge bot last polled the queues after polling all registered repositories.

Written by the indexing model from the issue text.