[FIX] web_editor, website: prevent multiple click on save

Clicking on save on html editor is taking quite some time. The user might think
something is not happening and need to click again.
If he does click again, 2 write RPC will be sent, making the COW business code
being triggered twice, ultimately leading to create 2 COW.

Instead of
  A
 / \
B   B'

It will be
    A
  / | \
B   B' B'
With as many B' than there is click on save.

This commit adds a generic helper to make a button load/spin.

closes odoo/odoo#63417

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>