Add aria-describedby for md-error and md-hint

[jelbourn]

We currently don't have anything that associates the content of md-error and md-hint with the input. We should generate IDs for these elements and point to them on the input's aria-describedby.

We'll need to be sure not to clobber any existing describedby when adding/removing these IDs.

We should also test out whether we should add aria-hidden to the md-error and md-hint elements so that they aren't repeated in an annoying way.

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