fix: special characters in editor

[Stef-Rousset]

🎩 Description

This PR prevents adding images with special characters in editor.

Testing

1. As an admin, go to a process, create a new proposal. See that you have text warning that you should not have special characters in the name of the file. Try to drop an image with a special character in its name (like test(image.jpg or test-image.jpg) and see that your image is not loaded and you have an error message (screenshot one).
2. Try to load the image with the upload modal, see that you have a text "No special characters.." and an error when trying to add an image with an incorrect name (screenshot two).
3. Try the to add an image with a valid name by dropping it or with the modal, See you don't have an error and your image is loaded.
4. Do the same tests in a process where you have added public proposal custom field of type textarea/rich-text-editor via decidim-awesome (screenshot three), and see that you have the same behavior.
5. Do the same tests as a user in the FO for a normal proposal and a proposal with custom field, and see that you have the same behavior.

📌 Related Issues

Link your PR to an issue

• Fixes https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=150585351&issue=OpenSourcePolitics%7Cintern-tasks%7C331

Tasks

• Add specs

📷 Screenshots

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