[IMP] sign: show one line header while signing

One line header logic
show/hide priority: Company Log >> current_signer[signature] >> other_signers >> Odoo Logo

Large:  Company Logo            Odoo Logo               current_signer[signature], waiting_signers, canceled_signers, refused_signers, completed_signers[signature]
medium: Company Logo            current_signer[signature], waiting_signers, canceled_signers, refused_signers, completed_signers[signature]
small:  Company Logo            current_signer[signature]

In when the width of the window is smaller than a fixed number, all [signatures] will be hidden for the sake of the class mobile-hide

Details:
remove `Sign Document` button and reuse `Validate & Send Completed Document` button when there is no sign item in the document.
for backend signers, move `Ctrl + Left click to add a field` to the right of action buttons of the control panel
for backend signers, move signer status info to the pager of the control panel

Notes:
In order to keep the maximum responsiveness,
this implementation requires current_signer[signature] to be the first one in the signer list, which is different compared with the past.
(old behavior: is_signing_signer, waiting_signers, canceled_signers, refused_signers, completed_signers[signature])
Restore it if you can:)

Before this commit:
While signing, signers will see multiple-line header which occupies too much space of the signing PDF(document)
It is better to show only one line header

taskid:2735852