Update Schema to disallow lists inside of paragraphs

[goneall]

Some license HTML files are being flagged as invalid due to lists being included inside paragraphs in issue #1680

From this Stack Overflow article, it looks like lists should not be allowed inside paragraph tags.

Note that this will be a slightly messy update to the schema since the current paragraph tag is specified in 3 different parts of the schema and re-use type information.

[zvr]

Right, in HTML lists cannot be inside paragraphs.

One may omit the end tag of a paragraph, but it is considered to be closing when a list-begin tag (<ul> or <ol>) is encountered.

[jlovejoy]

@swinslow @goneall @zvr - moving this to "later release" as it's been open for a long time... is there an action item here?