I updated all the necessary `help_text` arguments on the form. I
performed few changes in the template to have a better rendering such
as:

- add a margin between the form and the sandbox box
- remove the floating button for the submit button (prevent an overflow
  of the footer on top of the button)
- add a "no preset available" text in the drop down menu

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>