Draft and lifecycles directories cleanup

[nventuro]

Part of #2060

This PR is best reviewed commit by commit.

It achieves the following:

• Moves Pausable into utils, removes lifecycles
• Moves Strings and Counters from drafts into utils
• Moves SignedSafeMath into math
• Moves ERC20Snapshot into ERC20, makes snapshot an internal function
• Removes the drafts directory
• Adds a deprecation notice for drafts

OpenZeppelin/docs.openzeppelin.com#160 creates the required redirects.