Complete Netlify setup

[chalin]

The Netlify setup, at least with respect to GitHub, isn't complete. For example:

• We don't currently get build previews
• The production build isn't minimized, which affects layouts

Prep

• Rename master branch to main #151 - why not!

Tasks

• Set version to 0.7.0-dev, adjust default branch, and more #213
• Set Netlify build commands via netlify.toml #214
• Netlify config: drop NPM prepare command #215
• Drop themesDir from NPM script, tweak README, add dict #219

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We now have easily accessible build previews!

p.s. It seems like we had this enabled/activated several months back, but it stopped working. Not sure why.

[chalin]

Done