API: Overhaul token handling

The current token handling is a mess. This simplifies things greatly:
* *All* tokens are obtained from action=query&meta=tokens, rather than
  being spread over action=tokens, action=query&prop=info,
  action=query&prop=revisions, action=query&prop=recentchanges, and
  action=query&prop=users. All these old methods are deprecated.
* Similarly, there is only one hook to register new token types. All old
  hooks are deprecated.
* All tokens are cacheable.
* Most token types are dropped in favor of a 'csrf' token. They already
  were returning the same token anyway.
* All token-using modules will document the required token type in a
  standard manner in action=help and are documented in machine-readable
  fashion in action=paraminfo.

Note this will require updates to all extensions using tokens.

Change-Id: I2793a3f2dd64a4bebb0b4d065e09af1e9f63fb89
Merge "API: Overhaul token handling"