Linting: Add more HTML linting

[sinclairnick]

Currently, the ESLint plugin is largely automatic, based on the caniemail.com data.

However, there are more nuanced HTML rules that should be added, such as:

• [ ] Prefer using <div role=“”> over unsupported semantic tags like <header>
• [ ] Prefer html attr for width, height, align, valign, border, color or bgcolor (overloading ok)
• [ ] ...prefer styles for rest
• [ ] When using percentage widths, we should also set an html attr of width as fixed for OL support
• [ ] Border="0" cellspacing="0" cellpadding="0" should be set on tables for consistency
• [ ] Use margin and padding for spacing, multiple BRs and empty TDs should be avoided
• [ ] Prefer Hex color (with 6 characters?)
• [ ] Image should have alt tag
• [ ] Image should have src
• [ ] (Warn only) Consider setting a background color when setting a background image

More items will be added to this list over time.