appveyor and `vs2015` deprecation

To Do

• update to vs2017 (https://github.com/conda-forge/conda-forge-pinning-feedstock/pull/501)
• bump default in appveyor to vs2017 (https://github.com/conda-forge/conda-smithy/pull/1278)
• update the vc feedstock (https://github.com/conda-forge/vc-feedstock/pull/11)
• figure out if we need to exempt any feedstocks (https://github.com/conda-forge/feedstock-outputs/pull/2)
• need to turn off all appveyor as many feedstocks as possible to prevent new appveyor usage (https://github.com/conda-forge/admin-migrations/pull/18, https://github.com/conda-forge/admin-migrations/pull/21)
• need to remove appveyor from the docs and make an announcement item (https://github.com/conda-forge/conda-forge.github.io/pull/1010)

Feedstock Census

See notes here: https://hackmd.io/@beckermr/rJPw0tnDL/edit

[isuruf]

From the announcements page

We are formally deprecating vs2015 in two weeks on 2020-04-07 and will move to vs2017. This change will enable us to support the usage of msbuild on Azure for the win platform and will provide additional support for C++. Most packages built with vs2015 can be linked with vs2017 toolchain (but not vice-versa). An exception is static libraries compiled with whole program optimization (/GL flag) which may be incompatible with the vs2017 toolchain. These static libraries will need to be rebuilt using vs2017.

[beckermr]

I think overall the new compiler is good to go.