Link tasks to study when they're added

Problem

When a task is added to a study from the library (or a task is duplicated), a CustomizedTask is created in the db
It's not until the study is saved that the task bank is synced to the db
As a result, if you create a customized task and leave the page, it never gets linked to the study and is lost forever (just floating in the db with no reference to it)

When a task is added to the task bank via the library or duplication (save as), link it to the study in the DB
What this means is add the task to the study's batteries field in the following 2 endpoints:
[x] 1. the endpoint to create a customized task (POST /tasks/:id/custom)
- The :id in this endpoint is the id of the Battery, i.e. the task template, to create a customized version of
- You'll need to provide the route with the study id somehow. Not sure what the best way is, but these could work:
- Change to POST /tasks/:id/custom?studyId=xxx
- Change to POST /studies/:studyId/tasks?taskId=xxx where taskId is the id of the Battery
[x] 2. the endpoint to save a task (PUT /tasks/custom/:id)
- The :id in this endpoint is the id of the CustomizedBattery
- You'll need to provide the route with the study id somehow. The RESTful way is probably to change the route to PUT /studies/:studyId/tasks/:taskId
- Keep in mind that this endpoint is also used to save changes to already-existing tasks. We only need to add the task to the study if it's new
- For an example of this pattern, see the route PUT /studies/:id, which has differing logic if the study is new
```
const oldStudy = await Study.findOne({ _id: req.params["id"] });
...
const study = await Study.findOneAndUpdate(
  { _id: req.params["id"] },
  req.body,
  { upsert: true, new: true }
);
if (!oldStudy) await user.updateOne({ $push: { studies: study._id } });
...
```