Display load indicator during validation

• As a user
• I want to see that the validation is in progress
• in order to avoid clicking on "Validate" button again and again

Technical steps

While the "POST" request is ongoing:

• Disable the "Validate" button of the current tab
  • Do not disable the buttons of the other tabs (URL, Examples)
• Change the text to "Validation en cours..."
• Maybe use https://spin.js.org/