Add support for notes-for-users in the custom section of Boutiques descriptor

[prioux]

I'd like the Boutiques integrator to show some notes in bullet-point format at the top of a form when launching a task.

The notes would come from the custom section of the descriptor:

"custom": {
  "cbrain:input_notes": [
    "Please leave the field number of threads (--threads) blank, it is not used in CBRAIN",
    "Do not enter a maximum memory (-MAX_MEN)"
  ]
}

which would simply render as these notes at the top of the form, something like this:

---

CBRAIN integration notices:

• Please leave the field number of threads (--threads) blank, it is not used in CBRAIN
• Do not enter a maximum memory (-MAX_MEN)

---

[MontrealSergiy]

@prioux @natacha-beck is it still relevant? Value fixer addresses use case of --threads