kill off JuliaWebRepl and nginx

this deletes everything JuliaWebRepl related, for #2422

it is now hosted at https://github.com/vtjnash/JuliaWebRepl.jl

I am now installing uv.a, uv.h and julia.h, since the libuv files are needed to build the webrepl

(contrib/mac-gtk.sh slipped in to this commit and is neither good nor bad imo, just unrelated)