git-lfs: Added an optional apiurl parameter.
This needs version 1.2.5 of the haskell git-lfs library to be used.
stack.yaml updated to use that.
Note that git-annex enableremote can be used to add apiurl= to an existing
git-lfs special remote. To allow unsetting the apiurl and instead use
the probed url, support enableremote with apiurl set to an empty string.
Sponsored-by: Luke T. Shumaker
