Update express

[hampelm]

We're on 3.0.x; the latest 3 version is 3.21.2.

3.x is officially deprecated by now; looks like there are a lot of changes in 4: http://expressjs.com/guide/migrating-4.html

[prashtx]

We can use LocalData/sensors as a reference when upgrading, for example when selecting 4.x-compatible middleware.

[hampelm]

👍 I'm working on getting us to 3.21.2 and then will go for 4