minor: form feedbacks

Closes #710 (closed) #529 (closed) #781 (closed)

Contains:

• #575 (closed) Order on skills select, partially blocked by framework/sib-core#668 (closed) - will not need any action on our side after the core fix
• #710 (closed) Toast feedback on form success
• #710 (closed) Required fields coherence (Star only when required)
• #710 (closed) Validation on username, email, website fields
• #529 (closed) Do not display the icon on missing profile field
• #781 (closed) Fix for the link to Make Hubl Better on dashboard
• Adapted tests for these new behaviors
• Fix for #722 (comment 38505)
• Fixed colors on "A Propos" to follow the app theme
• Sentry integration (untestable)

I had no way to provide feedback on error after submitting a form, but with required and validations I also had no way to reach this case now.

As usual, please use CTRL+SHIFT+F5 when testing, to ensure that you have nothing in cache.