[feature] Enable basic video support (mp4 only)

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This pull request implements video support (mp4) in GtS.

The implementation uses https://github.com/abema/go-mp4 to parse height + width of mp4 videos, and then generates a solid-color thumbnail for the video using those dimensions.

It's basically just an adaptation of #1165 so credit goes almost entirely to @theSuess !! I only adapted it so I could tidy it up a bit, fix some tests, and remove the ffmpeg option to focus solely on the basic go implementation.

closes #1089

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.