Add astronomy to lesson dropdown
Add astronomy to lesson dropdown

For merging with publication of blog post announcing astronomy publication. 
- Adds astronomy to drop down
- Alphabetizes curricula
- Removes semester materials from drop down - we don't offer these and should probably eventually migrate to the Incubator.  . .
datacarpentry/datacarpentry.github.io,datacarpentry/datacarpentry.github.io,datacarpentry/datacarpentry.github.io