[w3c/uievents] Expose legacy IDL blocks (#287)

Closes #286 

The following tasks have been completed:

 * [x] Confirmed there are no ReSpec/BikeShed errors or warnings.

This follows #272.


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### :boom: Error: 500 Internal Server Error :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on Dec 17, 2020, 11:32 PM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [Spec Generator](https://www.w3.org/2015/labs/) - Spec Generator is the web service used to build specs that rely on ReSpec.

:link: [Related URL](https://labs.w3.org/spec-generator/?type=respec&url=https%3A%2F%2Frawcdn.githack.com%2Fw3c%2Fuievents%2Fdb0ee1e1c3d33189b778bfea1c4f9a3d44c55b2d%2Findex.html%3FisPreview%3Dtrue)

```
🕵️‍♀️  That doesn't seem to be a ReSpec document. Please check manually: https://rawcdn.githack.com/w3c/uievents/db0ee1e1c3d33189b778bfea1c4f9a3d44c55b2d/index.html?isPreview=true&publishDate=2020-12-17

```

_If you don't have enough information above to solve the error by yourself (or to understand to which web service the error is related to, if any), please [file an issue](https://github.com/tobie/pr-preview/issues/new?title=Error%20not%20surfaced%20properly&body=See%20w3c/uievents%23287.)._
</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/uievents/pull/287


-- Commit Summary --

  * Remove [[!DOM-Level-2-Events]] to fix bikeshed fatal error
  * Expose legacy IDL blocks

-- File Changes --

    M index.html (2727)
    M sections/event-types.txt (18)
    M sections/legacy-event-initializers.txt (38)
    M sections/legacy-event-types.txt (4)
    M sections/legacy-key-attributes.txt (2)

-- Patch Links --

https://github.com/w3c/uievents/pull/287.patch

https://github.com/w3c/uievents/pull/287.diff


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