Add support for spoilers on images (MSC4193)

[gingershaped]

Description

Adds support for MSC4193 by allowing users to mark image attachments as spoilers, which will blur them in the UI until explicitly revealed. The m.spoiler.reason field is supported for received images, although this PR does not add a way to set it for sent images.

Fixes #1966

Type of change

• Bug fix (non-breaking change which fixes an issue)
• 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:

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

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[gingershaped]

If the most recent message in a room's history is a spoilered image which has been revealed, it will be re-hidden when the next message is sent. I'm not sure why this happens, but it seems to be a symptom of a bug which is out of the scope of this PR to fix.

[gingershaped]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

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[ajbura]

Apart from the request changes, We should not delete list.ts as it is a generic atom file which can be used somewhere else, or even better maybe we can change in "PUT" action to update the existing entry if it already exist.

[kfiven]

Thanks!