[FIX] web: ensure one2many field destruction on button cancel

The destruction of one2many fields is forced with the event change:effective_readonly
This revision add the forced destruction for cancel(discard) button as well

Otherwise, one2many fields are not properly destroyed when hitting the button "discard" (from save or discard).
This can be problematic for one2many editable list views (such as invoice lines) if you discard while having a mandatory field not filled in the invoice line: You can't recreate an invoice, the one2many editable list is messed up

Use case: Create an invoice, create a line, leave the description, required field, empty. Then, discard. Then, click on create.

opw-616946
[FIX] web: user lang has the priority on partner lang

Potentialy, the timezone too.

On item action click (such as menus in More.. and Print..), the data in form view had the priority on user context (through the sidebar_eval_context)

Therefore, if a field "lang" was present in the form view (like in partner form), the web/action/load xmlrpc call was using the partner language instead of the user language.

Example of wrong use case before the fix:
 - Set the user language in French, then go to a partner form of a partner with English set as language
  - Click on any button of the partner form, such as the "Invoices" button, notice that the last item of the breadcrumb is in English, instead of Frenh (the user language)
  - Click on any menu opening a wizard in the More.. dropdown menu, notice that the wizard title is in English instead of French
  - Print any report from the Print dropdown menu, notice that the report file name is in English. If you print the same report for the same partner but from the list view, the report file name is in French.
[FIX] email_template: default model for save as new template feature

When sending a message with the "Compose new message" button on the right of the user menu, in the top bar, if you tried to save the message as template, you had a traceback because model field of email.template is mandatory, but was set to True because there is no model in such a case.

As there is no any relevant model in such a place, and that the field is mandatory, mail.message is pretty convenient as the default value.
[FIX] email_template: missing translations in email template

When composing an email based on an email template, some parts of the template
(the result of name_get on fields) were not translated.
This was due to missing language in context when rendering the template.
Fixes #3708, opw 617309
[FIX] account_aged_partner_balance: accurate total

In some cases when the move was partially reconciled, the amount of the move wasn't added to the total
This is related to rev. abe5c803a0e36f18aec4cc56abc289f66329a749
[MERGE] forward port of branch 7.0 up to db98434
[MERGE] forward port of branch saas-3 up to e11d1c2