Return the current billing period with subscription metadata

The usage page can show how much of a plan has been used, but not when the allowance resets. The billing period is never returned, so there is nothing to put on screen.

Return the current period's start and end with the subscription metadata the page already fetches. Null where there is no billing term, which the page already handles.

Depends on #6099, which adds the calculation.

**Done when** the page can show which period someone is in, when it resets, and how long is left.