Make reset password optional

[jordisala1991]

Feature Request

It would be nice to enable / disable reset password and avoid dependencies with mailer.

Extracted from: #1465

[Hanmac]

I'm currently brainstorming about this one, where i try to think of the possible constellations and how they should react:

• first it should check if there are resetting values, if not, then a big chunk doesn't need to be configured.
  • there doesn't need to be reset routes, and the link in template doesn't need to be there either
  • without resetting there doesn't need to be sonata user mailer, and also no symfony mailer
• then should it be possible to disable the resetting by disabling the sonata_user.mailer config?
• when having a custom mailer, then it doesn't need to complain about symfony mailer

any other should complain about missing services, right?

and if the resetting should work without admin bundle