[MERGE] website, *: lazy load most frontend javascript

* web, survey, account, im_livechat, payment, web_tour,
website_sale, base

Before this PR, the asset bundles were loaded in a synchronous way,
so website pages appeared to the user when all the css *and* js were
downloaded, parsed and executed. This PR adds a series of
improvements to enable lazy loading the JS on the frontend. Especially:

1) Extract the minimum part of the assets common:
- promises library and polyfill
- boot.js

2) Create "assets_common_lazy" which is "assets_common" - "minimum"

3) Include the "minimum" assets in frontend layouts using "defer"
loading and "assets_common_lazy" using "lazy" loading

4) Create a script to do the actual lazy loading in a "minimum" frontend
assets

5) Lazy load the current frontend / website editor assets

See sub-commits for details.

task-1961045

closes odoo/odoo#32181

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

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