[FIX] point_of_sale: copy/paste error, super must use the apropriate class
[FIX] account_voucher: avoid error if no partner

The field partner_id is not required on an account.voucher but the validation was failing if none was set (opw 611663).
This patch makes a fallback on the account of the voucher if neither a partner nor a writeoff account is specified.
[IMP] account_voucher: make the field tax_amount always readonly

The field tax_amount is fieled with onchanges and the compute_tax method. Setting a different value than the one computed by the system may lead to unbalanced move (which is obviously wrong).
In the future, handeling these operations by setting the correct value to the tax accounts would be better.
[IMP] Cache bundles in ir.attachments instead of LRU
[IMP] website_quote: handle quote and website descriptions

Add field quote_description on product.template and turn website_description from sale.quote.line into a related.
Overwrite write and create methods to synchronise both fields when setting a product.
website_description on product.template is added by website_sale but there is no dependency relations between these modules so we need to define it in both modules.
[FIX] sale: define sale_note field before using it as a default value for sale.order

When uninstalling/installing sale module, this can be problematic
[IMP] Cache bundles in ir.attachments instead of a LRU

Forward ported from 8.0's commit: 8402625
[FIX] account_voucher: wrong type in context

The type 'general' no longer exists on a account.voucher. As in general context, removed the default value on this menu.
Fixes #1261, it produces a traceback in 8.0 where the selection fields are less fault-tolerant.
[IMP] product: specify a view for 'Variant Values' action
[FIX] sale_layout: store=True is causing trouble when generating invoices from a pos order
[FIX] models: unexpected missing value when checking for a field in cache

At the end of _prefetch_field(), a check is made to ensure that the cache
contains something for the field to prefetch. The check was incorrect because
the cache was checked for a regular value only.
[FIX] models: improve rationale for the management of flag 'recompute' in context

When the context contains 'recompute': False, the recomputation was not even
prepared. Now both create() and write() prepare the recomputation by invoking
method modified(). The flag only controls whether method recompute() is invoked.
In addintion, the former flag 'no_store_function' was converted to the flag
'recompute', so that both create() and write() use the same flag.

Fixes #1456
[FIX] web: rendering issue in mail composer on Firefox

The cleditor width does not include the margins. Setting 100% will make the editable area too large (104%) on Firefox (opw 611700).
Replaced by auto, the default value adviced by CLEditor.
[FIX] purchase: purchase buttons on product form only for purchase users
[FIX] mail: bad test
[IMP] hr: strenghtening of write()
Merge pull request #1790 from thanhchatvn/patch-2

Update vi.po
Merge pull request #1800 from blaggacao/8.0-l10n_co

8.0-l10n_co
[FIX] web: ignored readonly attribute on image widget
[FIX] mail: tell orm to rename column "read" to "is_read"