Create new API to delete multiple pieces of content

PUT /contents/delete

```
["3d0f40fa-1e1b-470a-8755-75a389e78505",
 "de536722-24c8-4410-a742-ba6cde5fa0c5",
 "6091f870-1dc6-42b2-9386-cb956061e869"]
```

Towards Connexions/webview#799

I made this a PUT request instead of DELETE because DELETEs are not supposed to have a request body.