feat(tabs): add theming and ability to set background color

[willshowell]

Fixes #1362
Fixes #2908
Fixes #5014

[willshowell]

I definitely need to rework this to cut down on how much css is generated

[ocarreterom]

How about something like:

$theme-colors: (
  primary: $primary,
  accent: $accent,
  warn: $warn
);

@each $name, $color in $theme-colors {
  &.mat-#{$name} {
    @include _mat-tab-label-focus($color);
    @include _mat-ink-bar($color);
  }

  &.mat-background-#{$name} {
    @include _mat-tab-label-focus($color);
    @include _mat-tabs-background($color);
 }

 &.mat-#{$name}.mat-background-#{$name} {
  @include _mat-ink-bar($color, default-contrast);
 }
}

[willshowell]

cc @andrewseguin

[jelbourn]

Review on this one might be delayed a bit (Andrew is wrapping up initial data table stuff)

[willshowell]

@jelbourn fair, thank you for the update

[andrewseguin]

Looks great, thanks for the thorough work and your patience for the review. One suggestion, can you rename the background input to backgroundColor? Also, you'll need to rebase

[willshowell]

@andrewseguin done, PTAL

[kara]

@willshowell Can you rebase again? Sorry!

[willshowell]

@kara done!

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