[FIX] models: display_name and name_get mismatch

- display_name uses name_get and not the other way around:
name_get should not call _compute_display_name, _compute_display_name should call name_get.
The previous behaviour was not backward-compatible with the old api.
All the models redefining name_get would have 2 different behaviors between name_get and display_name.

- Do not set an inverse function to display_name:
In most cases, writing on display_name writes on _rec_name (if any, not mandatory).
If the display_name computation is redefined, we need to redefine as well the inverse method to avoid unexpected behaviour
This required to also modify tests in base_import as readonly fields are avoided.

- Remove search method on display_name:
For the same reason as for the first point, it could be good that searching on display_name use name_search (and not the other way around).
However doing this would be very inefficiant (need to do the search, without limit, extract the ids of the name_get result just to generate
a subdomain ('id', 'in', [...]). As in most cases it would anyway mean to search on the _rec_name it's better to directly do so.

- Changing label to avoid mismatch:
In view displaying the list of fields or when a match is made on the label of a field (e.g. when importing csv file,
matching is made on both label and technical name), the fact that display_name field has '
Calling it 'Display Name' will avoid most errors.

- remove display_name definition from website_forum_doc,ir_model:
These fields are doing the same thing as the display_name of the new api, we can remove them.
We need to keep the one for res.partner as it's a stored field.
[IMP] account.invoice: add extra @id on `smart button` to make inheritance easier
[FIX] res.users: safer read() when computing default company value

The goal is to avoid any kind of prefetching
of other fields when copmuting the default
company, as this will sometimes happen in the
middle of a database update, when the default
company is used for setting the default value
of new columns. In that case the db schema
may not fully match the in-memory model, leading
to an SQL error.