[FIX] css fix in graph view

The field selection menu was not positioned correctly when there was
a long list.  (was not displayed at the position clicked)
[FIX] mail: when checking for possible messages to fetch, perform
a search_count on mail_message when the actual number of messages or their
ids is not necessary, to speedup the computation.
[FIX] [IMP] mail: mail_message: when checking that access rights are not
violated in _search, do it in SQL to speedup the query. Indeed doing it via search and
browsing the results to validate the various rules is quite costly.
[REVERT] commit 5e5c73e

Branch 8.0 is a **stable** branch. This mean no database schema
changes.
[FIX] crm, hr_recruitment, task, issue: fixed default priorities + order based on priority (desc for everyone now)
[IMP] models: move prefetching of records back to method _prefetch_field

The selection of records in cache for prefetching was moved to method
_read_from_database() by xmo at rev 785018cc in order to fix an access right
bug.  But this introduced an issue: to explicitly avoid prefetching, you should
use read() instead of browsing records.  We revert the change by xmo, without
reintroducing the bug (which apparently was fixed by another way).