Allow for time-limited tokens

Tokens returned from User::getEditToken will now include an encoded
timestamp (and therefore will be up to 16 bytes longer).

User::matchEditToken and User::matchEditTokenNoSuffix are gaining a
"maxage" parameter: when specified, only tokens with an encoded timestamp
within $maxage seconds will be accepted.

Change-Id: Ic4cf148cc1fa9d0561aac0bd2d68a09c7851896d
Merge "Allow for time-limited tokens"