Update date logic of opengraph and schema internal templates

[torrayne]

This is in response to my previous PR #8186.

• Removed date checking of PublishDate and Lastmod
• Merged duplicate IsPage blocks in opengraph.html
• Formatted template tags so they're easier to follow. There are a lot of nested blocks and keeping track of what's open and what's closed was extremely tedious. I hope the changes I made makes this easier.

Feel free to cherry pick the date changes and exclude the formatting change.

[bep]

Your template code looks clean and good. There is one thing I think we need to fix, though. I don't remember my exact comment in that other thread, but my main take on this is that

• We don't need to fallback on some other date if PublishDate is not set
• But PublishDate may still be zero (e.g. in cases where there are no date set on a given document)

So, given the construct above, I suggest just wrap it in a with, e.g.

{{ with .Lastmod }}<meta property="article:modified_time" content="{{ .Format $iso8601 }}" />{{ end }}

[torrayne]

Just so I'm on the same page. You mean I should individually wrap both the PublishDate and Lastmod?

[bep]

Yes, assume all dates can be zero.

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