Upgrade to BlueprintJS 5

[erickzhao]

The library hit 4.x stable major last month: https://github.com/palantir/blueprint/wiki/Blueprint-4.0

[aryanshridhar]

Taking this and will also migrate to Popover2!

[codebytere]

@aryanshridhar i assigned you - please let me know if you'd like me to remove assignment :)