Implement text search

[marvin-roesch]

This adds a new button to the top right which turns into a search field when clicked. Search is powered by lunr.js and indices for all languages are created at build time so they are readily available in production.

I've tried to make foreign language search work as well as possible already, but input from our translators would be appreciated. @lianzhao and @botanicaxu, I'd especially like your feedback on the Chinese search!

Changes can be viewed at this preview.

Closes #99

[lianzhao]

Great work @PaleoCrafter !!

I have tried your online preview in Chinese. It works in some cases, but failed in others😂

It's hard to tokenize Chinese. I don't have much experience in both Chinese tokenization and lunr.js. It seems that lunr.js has official Chinese support? I'll try https://github.com/MihaiValentin/lunr-languages/blob/master/lunr.zh.js in my local dev env .

[marvin-roesch]

FYI @lianzhao I'll merge the branch for now and we can work on improving the Chinese tokenization as we go, since I expect that to be an ongoing effort.

[lianzhao]

Hello @PaleoCrafter, I did some research about tokenize Chinese with lunr today, and found a bug in lunr.zh.js which prevent me from tokenizing Chinese correctly with custom dictionary.

I've send a PR to lunr team. With that fix and custom dict, I can make the search work on my local dev env.