Subscribe button and branding

The "Subscribe on Bluesky" button could probably just be "Subscribe" and "Unsubscribe". I don't think we actually need the "Bluesky" branding here and, if any icon, maybe the Sequoia icon. Commenting is still a bit tricky because you really are commenting on Bluesky currently, so maybe we leave that. And because that has an icon, at least having the option for an icon (maybe a CSS variable to hide either is warranted?) to sort of "balance" the aesthetics, so we use the tree icon for Sequoia.

Could also use "Subscribe on Sequoia" for the button. That is, after all, what they are actually doing. They might authenticate with Bluesky, but ultimate it's up to their PDS.

If you like the idea, I'd like to make this a separate PR. Get the pieces working in stages.