Allow vtt file extensions for interactive video

Problem/Motivation

Text Tracks with a .vtt file extension can not be uploaded for interactive videos.

This was also reported at https://h5p.org/node/658870, indicating that the problem needs a fix. The H5P settings page is not obvious, and the vtt file needs to be added by default.

Steps to reproduce

Create an interactive video and try uploading a text track with a .vtt file extension. An error message indicate that the file type is not allowed.

Proposed resolution

Add vtt to the default whitelisted file extensions in the H5P config.

Remaining tasks

Apply the patch below.

User interface changes

vtt now appears by default in the whitelist at /admin/config/system/h5p