If no progress had been set before, setting `startedAt` did not work and
it would always been set to `finishedAt` or `Date.now()`. Settings this
if any progress had already been recorded did work.
This fixes the problem so that setting `startedAt` it properly works in
both cases.
This patch is a minor extension to the update progress and batch update
progress API and allows you to specify `finishedAt` and `startedAt` when
updating a progress.
If not specified, both values are still automatically set to the current
time. If just `finishedAt` is specified, `startedAt` is set to the same
value.
Example API request:
```
❯ curl -i -X PATCH \
'http://127.0.0.1:3333/api/me/progress/li_ywupqxw5d22adcadpa' \
-H 'Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VySWQiOiJyb290IiwidXNlcm5hbWUiOiJyb290IiwiaWF0IjoxNjY4OTYxNjAxfQ._NbilCoFy_hfoqy7uvbV4E_0X6qgLYapQ_MoRptP0oI' \
-H 'Content-Type: application/json' \
--data-raw '{"isFinished":true, "finishedAt": 1668556852000, "startedAt": 1668056852000}'
```
