This cuts down the time it takes to sort issues of a milestone by about
10x. In the previous setup the code would run a SQL query for every
issue that had to be sorted. The new setup instead runs a single SQL
query to update all the given issues at once.

The attached benchmark used to run at around 60 iterations per second,
using the new setup this hovers around 600 iterations per second. Timing
wise a request to update a milestone with 40-something issues would take
about 760 ms, in the new setup this only takes about 130 ms.

Fixes #3066