[FIX] models: stored func fields computation on one2many write

On one2many fields writing in an existing record (not when creating new record), the computed stored fields of the one2many were not re-computed correctly
Computed stored fields were marked as modified only if no_store_function was not set to True in the context. no_store_function is set when writing one2many records on an existing record.
Now, these computed stored fields are marked as to be recomputed, but are recomputed later, at the end of the existing record write
[FIX] website_forum_doc: fixed tree view impossible to use to create new chapters (missing forum_id due to wrongly copy-pasted views) + fixed max width of images displayed in the doc.
Merge branch '8.0' of github.com:odoo/odoo into 8.0
[FIX] point_of_sale: copy/paste error, super must use the apropriate class
[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] 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