fix(datepicker): error message alignment in md-input-container

[crisbeto]

Fixes the datepicker error messages not being aligned with other md-input-container instances. The issue was caused by an unnecessary float on the input element and the calendar icon pushing down the messages. The fix removes the float and pulls the icon up with a margin while pushing it down via top.

Fixes #9342.

Before:

After:

BREAKING CHANGE:

CSS md-date-picker: Prevents the button from wrapping around, as well as it pushing down the error messages more than they should be.

.md-datepicker-button { 
  top : $button-left-right-padding * 2 - $md-datepicker-border-bottom-gap / 2;
}

.md-input {
  float: none;
}