Add a spell checker

[freakboy3742]

What is the problem or limitation you are having?

We don't currently have any protection against spelling errors, and #618 demonstrates that spelling mistakes are slipping through.

Describe the solution you'd like

As part of a CI pass, we should be spell checking all content on the site.

Describe alternatives you've considered

Do nothing

Additional context

I don't know what (if any) options exist for Lektor - a quick search hasn't revealed an obvious plugin, but it might be possible to build one.

Alternatively, spell checking the final content might be an option.

[johnzhou721]

Using codespell has very few false positives, although we will need to filter out the results for foreign languages. I used codespell content | grep -v '+..\.lr:' | grep -v 'beefore' | grep -v '\-de\.lr' -- -de.lr is for a specific file that is in a foreign language that I forgot the name of.