Add "bot passwords"

Add "bot passwords"

Bot passwords are something like OAuth-lite, or Google's application
passwords: the bot can use API action=login to log in with the special
username and password, and will then be allowed to use the API with a
restricted set of rights.

This is intended to provide an easy migration path for legacy bots and
for bots on wikis without OAuth, since AuthManager is going to greatly
complicate non-interactive authentication. If OAuth is available, an
owner-only consumer would be a better choice.

Bug: T121113
Change-Id: Iaa4015e00edbfbfaedcc8b2d27a2d3fd25009159
Depends-On: I7e15331efb162275c4116bcae61f19d6b884cbe3