ExtensionHooks have always been pretty simple. Constructing them will

initialize them, shutting down will permanently destroy them, and

subclasses only needed to override __init__() and shutdown(). This was

almost too simple.
Once destroyed, a hook had to be re-constructed in order to be

re-enabled, invalidating any signal connections that may exist or other

registered data. Extensions that needed finer-grained state management

had to do it entirely within the hook through its own custom

enabled/disabled states, which still left them initialized and

registered on the extension.
This change improves this in a couple key ways. First, initialization

can now live in initialize(), which does not need to call the parent

method (unless inheriting from another ExtensionHook that defines

initialize()). The shutdown code in shutdown() no longer needs to call

the parent method either.
ExtensionHooks can be disabled by calling disable_hook(), and then

re-enabled (with new state) by calling enable_hook(). The instance will

remain around, but the hook's registration and state will be properly

handled. The hook can also be instantiated with start_enabled=False to

create the hook instance without enabling it, allowing the extension to

do that later in its process.
Along with all this, I've fleshed out the docs quite a bit.

Djblets and Review Board unit tests pass.