*: test_website, base, web_editor, website, website_blog, website_event,
   website_event_track, website_payment, website_sale, test_assetsbundle

Before this commit when a module was updated all ir.asset records were
reset to their defined `active` state, if defined.
This causes assets related to old snippet versions to be made inactive
even if those old snippet versions are used inside existing pages.

It used to work when the activation of assets was made through view
inheritance because when views are defined through a `<template>` tag,
the `active` attribute is in fact ignored during updates since [1],
except for new records since [2].

This commit introduces an `<asset>` tag in the XML import format.
It is an alias of `<record ... model="ir.asset">` with the additional
feature that it avoids taking the `active` field into account during
updates for existing `ir_asset` records, just like `<template>` if the
`active` field is mentioned as attribute of the tag.
We then rely on the `website_disable_unused_snippets_assets` cron to
properly disable any unused asset at a later stage (note that the bug
being fixed here was mitigated by the fact that cron also re-enabled
assets which were disabled by mistake... but that might happen only a
few days later).

Another approach was to overload `_load_records_write` in `base`'s
`ir_asset.py` to avoid taking the `active` field into account when
updating records:
```py
    def _load_records_write(self, values):
        values.pop('active', None)
        super()._load_records_write(values)
```
But this is not as stable because it changes the way `ir.asset` records
are imported when the `<record>` tag is used. In the end we chose to be
consistent and do exactly the same as `<template>`, as this also allows
more and should be entirely stable.

[1]: 2d296cb#diff-175c28787c272a219b9275f79262a48af9aa029e718f45077fd609737559e84eR803-R804
[2]: f1c70d4#diff-175c28787c272a219b9275f79262a48af9aa029e718f45077fd609737559e84eR801-R804

task-2963840

closes #235124

X-original-commit: ae773c6
Related: odoo/enterprise#99190
Related: odoo/documentation#15215
Related: odoo/design-themes#1177
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>

*: theme_anelusia, theme_artists, theme_avantgarde,
   theme_aviato, theme_beauty, theme_bewise, theme_bistro,
   theme_bookstore, theme_buzzy, theme_clean, theme_cobalt,
   theme_enark, theme_graphene, theme_kea, theme_kiddo,
   theme_loftspace, theme_monglia, theme_nano, theme_notes,
   theme_odoo_experts, theme_orchid, theme_paptic,
   theme_real_estate, theme_test_custo, theme_treehouse,
   theme_vehicle, theme_yes, theme_zap

See community commit for details.

task-2963840

closes #1177

X-original-commit: 69d58de
Related: odoo/odoo#235124
Related: odoo/enterprise#99190
Related: odoo/documentation#15215
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>

Also removes outdated things in the `<template>` tag documentation and
reviews it a bit.

See odoo/odoo#104836

task-2963840

closes #15215

X-original-commit: f7777d9
Related: odoo/odoo#235124
Related: odoo/enterprise#99190
Related: odoo/design-themes#1177
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>