Fix Relative Path Traversal Attack on note creation

Impact
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An attacker can read arbitrary `.md` files from the server's filesystem due to an [improper input validation](https://cwe.mitre.org/data/definitions/20.html), which results in the ability to perform a [relative path traversal](https://cwe.mitre.org/data/definitions/23.html).

CVSSv3 string: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

PoC / Quicktest
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To verify if you are affected, you can try to open the following URL: `http://localhost:3000/..%2F..%2FREADME#` (replace `http://localhost:3000` with your instance's base-URL e.g. `https://demo.hedgedoc.org/..%2F..%2FREADME#`).
- If you see a README page being rendered, you run an affected version.

Analysis
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The attack works due the fact that [the internal router, passes the url-encoded alias](https://github.com/hedgedoc/hedgedoc/blob/master/lib/web/note/router.js#L26) to the `noteController.showNote`-function. This function passes the input directly to [`findNote()`](https://github.com/hedgedoc/hedgedoc/blob/78a732abe691b496fa3692aa2add37f7344db1fa/lib/web/note/util.js#L10) utility function, that will pass it on the the [`parseNoteId()`](https://github.com/hedgedoc/hedgedoc/blob/78a732abe691b496fa3692aa2add37f7344db1fa/lib/models/note.js#L188-L258)-function, that tries to make sense out of the noteId/alias and check if a note already exists and if so, if a corresponding file on disk was updated.

If no note exists the [note creation-function is called](https://github.com/hedgedoc/hedgedoc/blob/78a732abe691b496fa3692aa2add37f7344db1fa/lib/models/note.js#L240-L245), which pass this unvalidated alias, with a `.md` appended, into a [`path.join()`-function](https://github.com/hedgedoc/hedgedoc/blob/78a732abe691b496fa3692aa2add37f7344db1fa/lib/models/note.js#L99) which is read from the filesystem in the follow up routine and provides the pre-filled content of the new note.

This allows an attacker to not only read arbitrary `.md` files from the filesystem, but also  observes changes to them.

The usefulness of this attack can be considered limited, since mainly markdown files are use the file-ending `.md` and all markdown files contained in the hedgedoc project, like the README, are public anyway. If other protections such as a chroot or container or proper file permissions are in place, this attack's usefulness is rather limited.

Workarounds
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On a reverse-proxy level one can force a URL-decode, which will prevent this attack because the router will not accept such a path.

For more information
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If you have any questions or comments about this advisory:
* Open an topic on [our community forum](https://community.hedgedoc.org)
* Join our [matrix room](https://chat.hedgedoc.org)

Advisory link
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https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-p528-555r-pf87

Signed-off-by: Christoph (Sheogorath) Kern <sheogorath@shivering-isles.com>

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