add webm to the default fetchinclude

[mansona]

Summary

Fixes #777

This PR adds webm files to the default fetchinclude. This means that users who are making use of LFS and have webm video files won't have issues when deploying sites 👍

I struggled for about a week trying to find the exact set of levers to pull to make LFS work with my site and I came across this thread that ultimately solved the issue: https://answers.netlify.com/t/webm-not-working-under-git-hub-lfs/14460/4

My reasoning for opening this PR is that I'm assuming (possibly naively) that this can't hurt anyone's existing sites, and it just unblocks people that happen to be using LFS with webm files.

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So I have made the only check that I have control over pass 🤔 I don't know how to make the label check pass since I'm not able to label this PR 🤔

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