Videos: Use start time and end time for clips (#4264)

This PR parses the start and end time for clips.

It also adds a new, dedicated API endpoint (`/api/v1/clips/{id}`) for
retrieving the start and end time of a clip.

Here is a sample response from that new endpoint (`video` is a video object,
as described in https://docs.invidious.io/api/common_types/#videoobject):

GET `/api/v1/clips/UgkxxPM3BRphCAPLP88YoUGuj79KXPfpNNO_?pretty=1`

Response:
```
{
  "startTime": 8842.645,
  "endTime": 8855.856,
  "clipTitle": "✂️ Kirby is pink!",
  "video": {}
}
```

Closes issue 3921

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