[FIX] web: wait for deferreds when actualizing mode

In edit mode, in a *2many with many2many_tags, when adding a new tag when none was set, discarding the form let the new tags displayed while it shouldn't (only a display issue, the tag wasn't added in database)

Fixes #2926
[FIX] email_template: do not try to add user signature if none set

append_content_to_html expects two strings. If the user has no signature, it returns user.signature returns False
[FIX] point_of_sale: fixed display of untaxed amount in sales lines report
[FIX] web: cherry-pick of 893e4268650ddd517e3044f41c582fd92de6edfe & 61207f3f4d9206d4c1a3e484eb89f09d2258977a