Add NoContentVerb

[fierce-katie]

Closes #1028.

The NoContentVerb endpoints always return status code 204 and empty body and it is reflected in the docs.

[fierce-katie]

The only failing build on Travis CI is the one for ghc-8.0.2. It fails with

internal error in InstallPlan.completed
CallStack (from HasCallStack):
  error, called at ./Distribution/Client/InstallPlan.hs:262:25 in main:Distribution.Client.InstallPlan

I have not changed the CI configuration.

Probably related to haskell/cabal#6060

[phadej]

I don't have an opinion on this

Probably this is a right thing to do now.

#841 should make this unnecessary, but it's a holy grail which would fix everything, yet not founded still :)

---

I think that we should remove NoContent type if we go with this. ping @alpmestan

EDIT: above doesn't need to be done in this PR; also I'll figure what's wrong with CI myself

[phadej]

OVERLAPPABLE/OVERLAPPIG (here and elsewhere) are not required, are they?

[alpmestan]

I like this patch, because users don't have to specify an arbitrary content type anymore.

[phadej]

Thanks. Merged rebased version in #1228 (CI was green there).