spellcheck

Since it is a pain to fix typos in the current website, this adds a spellchecker to the gitlab CI. It uses a built-in dictionary, then also includes all app names and JSON field names from the index file. There is a .spelling file to add words to be ignored by the spellcheck. It will only run the spellcheck on Markdown files that are included in a Merge Request.

This merge request includes changes to README.md and TRANSLATIONS.md. README.md now passes the spellcheck, and TRANSLATIONS.md still has one error left. So you can see the output. Ideally the spellcheck would ignore stuff in inline code blocks, like the error in TRANSLATIONS.md. I couldn't find a way to do that.