[FIX] Solve some mail issues which are currently present in 7.0
Merge pull request #724 from odoo-dev/7.0-mail_issues-fix-adh

[FIX] mail: misc bug fixing
- avoid deleting the Whole Company mail group if still present, because it is required for some module installation. In 7.0 installing some modules may crash if this group is not present.
- fixed an issue when computing possible recipients in the Chatter
- small fix in html2plaintext about blank lines
[FIX] orm: custom m2m with different label

At rev 84e9a67cdf78db94cb7a09543c1b7ac4ad19d8b4 a check to avoid the creation of ir.model.relation for custom modules was added. The condition is not correct as based on the string instead of the field name. We do not have access to column name at this level but the the m2m relation table do start with x_ for custom fields (see __init__ method).
[FIX] hr: call hr_employee super's when unlinking

Also fixes some weird inheritance bugs in upper versions
[FIX] event_sale: reset event-related fields on SO line when switching products
[MERGE] Fwd-port 7.0 up to 783b9e1