Allow videos to be marked as spoilers

[gingershaped]

Description

This PR updates Cinny's implementation of MSC4193 to support spoiling videos as well as images. (ref. https://github.com/matrix-org/matrix-spec-proposals/pull/4193#discussion_r1983773016)

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings

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[gingershaped]

@ajbura Is this pull request acceptable in its current form? It's been open for several months.

[kfiven]

No issues with this PR but can we make sure that this is actually proposed in MSC?

[gingershaped]

No issues with this PR but can we make sure that this is actually proposed in MSC?

It's in the MSC as of this commit.