Add an Atom feed icon on the tag page

[simonw]

The tag pages are getting really good now and have feeds at /tags/datasette.atom but they are not linked from the page.

[simonw]

It should show up on /tags/open-source/ but not on combined tag pages like /tags/open-source+google/.

[simonw]

[simonw]

I put a <title> tag in the <svg> which I think acts as equivalent to an alt= attribute.

[simonw]

Change is live, e.g. on https://simonwillison.net/tags/python/