Feature detect Symbol support. Use string as a fallback.

[ms]

IE11 does not support Symbols natively and it is tricky to polyfill them
in a spec-compliant way.

Instead of using a Symbol to the React object, we can use an equivalent
string that is unlikely to conflict to store the ApolloContext object.

This is similar to the strategy used in React itself: https://github.com/facebook/react/blob/7dc9745427046d462506e9788878ba389e176b8a/packages/shared/ReactSymbols.js

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[hwillson]

Thanks for working on this @ms!