Add type="button" to controls in mat-calendar

[DAquilina]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Clicking a calendar button should interact with the calendar and not the parent form

What is the current behavior?

Clicking a calendar button submits the wrapping form

What are the steps to reproduce?

1. Put a mat-calendar inside a form with a submit handler
2. Click next or previous

https://angular-material2-issue-zi9txa.stackblitz.io

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Tested with latest in ng2 (6.0.0-beta.7) and material2 (6.0.0-beta.4), as well as ng5.2.4 and material 5.2.1. I've been using OSX 10.13.3/Chrome but I guarantee that this will happen in any modern browser.

Is there anything else we should know?

By default, the type of a button element in HTML is set to "submit", so when it is clicked it will submit the form regardless of what click handlers get invoked.

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