Tooltip: Require onClickTrigger for learn more pattern

[interactivellama]

Fixes #1549
Fixes #1705

Additional description

• If learnMore Tooltip variant is used without onClickTrigger, then the “no click” basic info icon tooltip will be used with a “disabled” button as trigger.
• If onClickTrigger is defined, a link will be rendered (this is the current behavior for learn more tooltips)

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CONTRIBUTOR checklist (do not remove)

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REVIEWER checklist (do not remove)

• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.

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