bug(material/datepicker): Re-evaluate how the MatDatepicker handles timezones

#### Bug, feature request, or proposal:
Documentation request

#### What is the expected behavior?
The moment object returned by selecting a date with Datepicker should have no timezone information (or UTC)

#### What is the current behavior?
Currently the moment object looks like this: Wed Apr 18 2018 00:00:00 GMT+0300
(The user is located in GMT+3, so his timezone is used)

#### What is the use-case or motivation for changing an existing behavior?
I wonder how to make my matDatepicker return UTC dates so that they are independent of the timezone the user is currently in. I've found lots of issues regarding this (https://github.com/angular/material2/issues/7167), but no clear and simple explanation on how to easily achieve that.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 5.2.0, Angular Material 6.0.0-beta.4, Typescript 2.5.3, ANY browser (not just IE!)

#### Is there anything else we should know?
I'm importing MatMomentDateModule, so all the dates in the app are momentjs dates.

先从 MatDatepicker 和 MatMomentDateModule 文档开始，然后查看相关 issue #7167，了解现有的时区讨论。记录一种清晰且受支持的方式来获取与时区无关的日期或 UTC 日期，包括 moment 值的预期行为；当用户无需依赖 issue 中零散的评论即可遵循指南时，该 issue 即告完成。

由索引模型根据 Issue 内容生成。