Removes large pypi image in Slack link unfurling

[rmasters]

One of my pet peeves is that sharing a pypi link in Slack embeds a tall image - this displays at most two pypi links on my 13" screen until the expansion is deleted by the user.

This PR prevents inclusion of the og:image meta tag when the user-agent represents the Slack link unfurler. It keeps the existing images for Twitter and other social network embeds.

Slack	Twitter	Facebook

This PR also makes the following changes to the Twitter embed:

• Declares the card as a summary card, which was preventing some preview tooling from working. I believe this was previously falling back to a summary card by default.
• Adds @pypi as the site.

It's worth noting that if #2211 is implemented, this should be reverted, ideally with an image height constraint.

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Despite it's simplicity, I'm not sure if adding to the project/release view context is the best approach for this. Would you prefer this to be a service?

I'm considering some further expansion to this - adding fields like the github repo link, downloads count and latest release date to the metadata (as seen in this example, using the twitter:label1/twitter:data1 meta properties).