get_installed and _lang_get_id are both ormcached and correctly check
the context

Retrieving a res.lang from a code is a frequent action that can be
achieved with _lang_get (cf previous commit).
Using _lang_get ensure the active_test in the context is correct and
is not poluted with another context propagation issue.
odoo/odoo#35490 discussion is an example of bad context propagation

closes odoo/odoo#35504

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

Multi is the default api for methods, it is not necessary to explicitly
decorate methods with it, adds clutter and most people use it because
they see that the rest of the code uses it.

Done with `find . -type f -name '*.py' | xargs sed -i '/@api.multi/d'`

Migration of website module to new API.
The tricky phase is ir_http.py : we need to use `request.env`
only when the authenfication phase is done. Normally by
calling `super` of `_dispatch` method, but website module
required it to be done before. This is important since
`env`is a lazy property of `request` object.

Some hack were kept since this commit is a migration ('RequestUID'
in ir.http, ...)

Some docstrings were added.