[UI Framework] Button component needs to support "type" HTML attribute

[cjcenizal]

The KuiButton React component currently uses the type prop to specify whether it's a primary button, warning button, etc. We need to rename this prop so that people can use type to specify the HTML attribute, e.g. type="button" or type="submit".

Steps

• Rename the "type" prop to "buttonType". In ui_framework/components/button/button.js, this change will be on lines 66 and 96. Make sure that all of the references to this prop within the KuiButton definition are updated to the new name as well. Apply the same changes to KuiLinkButton and KuiSubmitButton in that file.
• Update the examples. In ui_framework/doc_site/src/views/button, update every example file here to use the new prop name. You can verify that your change has had the desired result by [running the UI Framework locally]
• Update the other examples. This component is used by other components in their examples, so grep the ui_framework directory for other instances in which the KuiButton, KuiLinkButton, and KuiSubmitButton React components are used, and update them with the new prop name.
• Update the tests. In ui_framework/components/button/button.test.js, on line 37, update the test suite description and the tests themselves to reflect the changes to this prop. Run the tests. The tests will fail, so just follow the command-line instructions for updating them with the new snapshots. Do the same for link_button.test.js and submit_button.test.js.
• Update the tests to assert that the "type" HTML attribute is supported. In that same file, for the test on line 21, add type="submit" as an attribute, make sure it gets rendered in the snapshot, and update the test. Do the same for link_button.test.js and submit_button.test.js.
• Update production code dependents. Grep the Kibana and X-Pack codebases for instances in which the KuiButton, KuiLinkButton, and KuiSubmitButton React components are used, and update them with the new prop name.

[cjcenizal]

CC @nreese