We would like to make the "no item found" screens more appealing.

Before this commit, it shows a small help tip in the top-left
corner of the screen, just below the "Create" button.

With this commit, these help tips have been replaced by onboarding
screens, which consist of a picture and some text below, both of which
are horizontally centered.

The texts have been slightly changed, so that they are shorter and clearer.

Considered modules:

(A)
    account,
    account_asset,
    account_budget,
    account_test,
    account_voucher,
    analytic
(B)
    barcodes,
    base,
    base_automation,
    board
(C)
    calendar,
    contacts,
    crm
(D)
    delivery
(E)
    event
(F)
    fleet
(G)
    gamification,
    google_drive
(H)
    hr,
    hr_attendance,
    hr_contract,
    hr_expense,
    hr_gamification,
    hr_holidays,
    hr_payroll,
    hr_recruitment,
    hr_timesheet
(I)
    im_livechat
(L)
    l10n_fr_sale_closing,
    link_tracker,
    lunch
(M)
    mail,
    maintenance,
    mass_mailing,
    membership,
    mrp
(N)
    note
(P)
    payment,
    point_of_sale,
    post_mercury,
    pos_restaurant,
    product,
    project,
    purchase,
    purchase_requisition
(R)
    rating,
    repair,
    resource
(S)
    sale,
    sale_timesheet,
    sales_team,
    stock,
    stock_account,
    stock_landed_costs,
    stock_picking_batch,
    survey
(U)
    utm
(W)
    web,
    website,
    website_blog,
    website_customer,
    website_event_track,
    website_forum,
    website_quote,
    website_sale,
    website_sale_digital,
    website_slides

Avoid duplicating web addon in enterprise by extracting a common basis.
Enterprise features stay in enterprise, but use that common basis.

Mainly:
 - JS refactoring and linting
 - Conversion of .sass into .less split into multiple files
 - Templates cleaning and DOM simplification
 - Re-generation of web.pot, and update of .po files