Update to node 8.x LTS

[gideonthomas]

Using node 4.x is becoming a pain for Thimble contributors who are trying to setup Thimble manually since Thimble and all other services it relies on (except login.wm.org) can run on node 6.x. Running nvm is not possible on Windows and its alternatives are pretty terrible.

[cadecairos]

Taking over this ticket so we can track the work we reverted in #396

@gideonthomas after merging that patch, staging started throwing an error trying to compare password hashes. Stack Trace in the revert comments