* [8.0][mass_mailing] some fixes
* [FIX] rename wrong mailing_type field and add missing fields when creating a mass mailing from the composer
* [FIX] coerce tooltip to an unicode string to avoid a json crash when locale produces a non unicode string for strftime(%B)
* [FIX] repair wrong sql statement computing statistics
[FIX] web: grid headers should be aligned properly
[FIX] fetchmail: do not delete a failing email when using POP

In case of IMAP, the email is simply set as read but in case of POP, the email is deleted. To avoid loosing data (e.g. misconfiguration of server), the email is kept for POP.
Bug lp:1296724, opw 605667
[FIX] product: don't display 'create and edit' option on many2many_tag to create variant on product_template. The quick create is confusing for users
[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).
[FIX] point_of_sale: generate bank statement with commerical partner

When a pos session is closed & confirmed, the account.move were generated with the commercial partner except for the bank statement which prevented automatic reconciliation.
This patch uses the commercial partner also for bank statement.
Fixes #1558, #1764
[IMP] stock: stock incoterm converted to uppercase
[ADD] doc: new documentation, with training tutorials, and new scaffolding
[IMP] doc/howtos/backend: use scaffolding and adapt text and code accordingly
[FIX] Make sure packages are not shown when the setting is not checked
[IMP] No propagation when destination move is done/cancel
Merge pull request #1776 from acsone/8.0-jne-mass_mailing-improvements

[FIX] Mass mailing: fixed statistics computation + summary graph tooltips (using ustr) + values of the mass mailing when creating it from the mail composer wizard. Courtesy of Acsone.
[IMP] doc/howtos/backend: add a small section on how to start the server
[FIX] board: Do not offer to create dashboard when there is no action

In case we have no defined action (e.g. redirection by python code), adding a dashboard should not be possible as the rendering based on the action.
This patch hides the 'Add to Dashboard' button in search view (opw 611288)
[FIX] mail: do not send email at module installation
[FIX] point_of_sale: the category list now uses less space when the categories don't have pictures, freeing some screen space on tablets
[FIX] model: prevent exporting column ID of non ordinary tables

This makes no sense and can generate errors if you try to purge
ir.model.data of not existing records.
[FIX] hr_holidays: employees cannot approve their holidays

Nor modify once approved

It wasn't possible for employees to approve their holidays themself, thanks to the GUI, but this was possible through xmlrpc calls, or when altering the html directly in the browser.
Besides, this was also possible to edit the holiday through the same trick once the holiday validated
[IMP] doc/howtos/backend: finalize section on translations, and other small fixes
[IMP] doc/howtos/backend: fix calendar example