Decide on how anchor-able components work

[dfreedm]

Related #328, #385

There are really only three ways to do anchors for some components:

Options 1: Anchor around component:

<a href>
  <mwc-button></mwc-button>
</a>

Option 2: Facade anchor attributes

<mwc-button href></mwc-button>

Option 3: Slotted anchor

<mwc-button>
  <a href></a>
</mwc-button>

We should pick one, and use it consistently

[abdonrd]

More references here: PolymerElements/iron-elements#37

[bennypowers]

The old polymer way was to wrap your UI in anchors. IIRC that decision was taken to stay in line with the rest of HTML, which is solid reasoning IMHO

It can get difficult when we're talking about nav links in a drawer which are also list items, and that's a very popular pattern