fix(popover): handle scrolling in content so header can be sticky

[averyjohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: resolves ionic-team/ionic-v3#102

What is the new behavior?

• Scrolling is now handled by the content inside the popover, rather than the outer popover viewport.
• Prevents the .inner-scroll in the popover's content component from overflowing said content's height, through display: flex in a couple places. This ensures the content scrolls properly.

Does this introduce a breaking change?

• Yes
• No

Other information

[liamdebeasi]

The layout looks a bit odd when translucent="true" is on ion-header and fullscreen="true" is on ion-content. The translucent effect does not seem to show up and the content at the bottom appears cut off.