bring 1.8.3 branch into git/trac

[warner]

The wiki:Doc page wants to point to the NEWS.rst file in 1.8.3, but 1.8.3 doesn't exist in our git history (I think it was made from a darcs branch that never got imported when we switched to git) (1.9.1 was similar, but *was* imported).

Since all we really care about is having the code *somewhere*, it'd be sufficient to make a git "rel-1.8.3" branch (from the 1.8.2 tag, probably), dump the whole darcs tree into it, add a single commit, and add the allmydata-tahoe-1.8.3 tag. Then update the wiki:Doc page to point at it's NEWS file.