fix(tables): correct placement of OverflowButton menus

[austingreendev]

Description

This PR updates header OverflowButton logic to simplify menu placement within a Table.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[jzempel]

@Austin94 the offset needs to be 1 or 2px higher (for both headers and regular rows) in order to match the intended layout shown by https://garden.zendesk.com/css-components/tables/.

[austingreendev]

Forgot about the default margin! I've now removed that and pre-published the updates to http://garden.zendesk.com/react-components/tables

[jzempel]

I'm gonna get really finicky here and show you what we want (I think you need to reapply a consistent negative offset to all menus in order to achieve the intended positioning). Please pay special attention to how high the top of the menu border is above the bottom border of the row (aka one pixel below the focused overflow trigger button).

INCORRECT HEADER ROW

CORRECT HEADER ROW

INCORRECT BODY ROW

CORRECT BODY ROW

[coveralls]

Coverage remained the same at 95.835% when pulling 8c03403 on agreen/tables-overflow-button into 5169523 on master.