Add more feeds to cover all subfields of astronomy & amateur astronomy

[emilyhunt]

Subfield-specific feeds allow for highly filtered views of posts on the Bluesky network. These subfield feeds will all crosspost to the main Astronomy feed, too - meaning that posts in subfield feeds won't be in isolation.

Outline

The 1.0 concept with these feeds will be to have:

• Coverage of all subfields of astronomy/astrophysics, including topics relevant to amateur astronomers
• About ~2 feeds per astro-ph category
• Around ~10 feeds in total

Additional features not currently planned include:

• Possible additional coverage of methods (e.g. a machine learning feed)
• Additional fun ones (e.g. "observe with me" for live-skeeting your observing night)

Existing feeds

• Astrosky - all posts by everyone in the Astronomy network (unfiltered)
• Astronomy - all astronomy posts

Planned feeds

Covering science:

• Cosmology
• Extragalactic astronomy (maybe just "Galaxies"?)
• Galactic astronomy (maybe just "The Milky Way"?)
• Exoplanets
• Planetary science
• High-energy astrophysics (is there a better / less technical name?)
• Astronomy Instrumentation
• Astrostatistics & machine learning (maybe...)
• Stars

I'm not sure whether more feeds (e.g. black holes, agn, star clusters) would be a good idea or if it would get too specific (and hence too diluted.) Heck, the above may be too diluted already...

Covering the more fun stuff:

• Astrophotography (probably good enough as a catch-all? Could also add a separate discussion feed and keep this just for pictures, though not sure)
• Space art (there may already be feeds that cover this, though)

[emilyhunt]

Exoplanets & astrophotography are now added!

I'll lay off on new feeds for a bit because they aren't getting so much uptake, and I don't want to dilute posts / add too much complexity.