Add a "save draft" action on notes

An ajax button could be displayed to save a draft Note, otherwise users might be afraid of losing their work in case their browser crashes for instance.

Issues to address:

• do not take versionning information into account, maybe display an alert before saving the draft in case a versioning option is checked (major or minor).
• html editor is not re-rendered correctly in ajax, see NXP-5578

Functional choices:

• log a specific event in audit logs? ignore comment for this log?
• display button above and below edit layout?

Feature should be easily available on other document types, but not available by default as for instance binary content cannot be uploaded in ajax for now.