Update wiki references (point to githib/wiki)
Merge branch 'master' into old-wiki-refs
Merge branch 'master' into remove-old-wiki-refs
Drop references to
https://github.com/sympy/sympy/wiki/old-wiki-Sympy-Development
Drop obsoleted refs to
https://github.com/sympy/sympy/wiki/old-wiki-Discussion
Merge branch 'master' into remove-old-wiki-refs
Merge pull request #1809 from skirpichev/remove-old-wiki-refs

Remove old wiki refs