fix(flow): fix editing content elements using action modal

[laurentlp]

This PR fixes an issue where when editing content elements (image, video, audio, file) it was possible to save an empty value.

Before

simplescreenrecorder-2021-09-21_17.57.28.mp4

After

simplescreenrecorder-2021-09-21_17.54.03.mp4

Closes #75 and DEV-1636

[linear]

DEV-1636 "[BUG] Updating Image Content Image UI issue" (botpress/studio #75)

Describe the bug

When updating url in an Image Content's Image input, you can click the Submit button without actually updating the field.

To Reproduce
Steps to reproduce the behavior:

1. Go to flow page
2. Create a node
3. Add an action, Image type, with image and title.
4. Go back to action, and change the image text. Click Submit, not OK (see the image)

Expected behavior
There should be a confirmation pop-up, or the submit button should be disabled (due to no changes being detected), or the change should go through.

Environment (please complete the following information):

• OS: macOS
• Browser Chrome
• Browser Version Version 92.0.4515.131 (Official Build) (x86_64)
• Botpress Version 12.25.0
• NodeJS 12.13.0

This causes frustration that happens when you think you updated something and you didn't.

---

botpress/studio #75 by @ptrckbp on GitHub

via LinearSync

[EFF]

LGTM