Several references to old removed option "bootswatch_theme"

[ChrisJefferson]

I spent a while trying to use "bootswatch_theme", as mentioned:

• https://themes.getnikola.com/v8/bootstrap4/

• https://docs.getnikola.com/en/latest/extending.html

• pages/extending.rst when I run nikola init --demo

However, it seems this option was removed, and replaced by nikola subtheme?

[Kwpolska]

Thanks for reporting this! I fixed all the mentions and redeployed the listed pages.