Move hooks sections together

Same change from 0.7 made on dev
Bump blaze-layout to 0.2.3.

See:
https://github.com/EventedMind/blaze-layout/commit/27d2a0e67a2636b85668abd929af9fc64c818356.

- More helpful error message for {{yield}}.
- Allow {{> yield "footer"}} in addition to {{> yield region="footer"}}
- Same for contentFor
- @tmeasday, can you take a quick look?
- cc @avital
Merge branch 'dev' into pr/536
Client helpers process args should default opts to empty object.

- Fixes #580.
Remove Handlebars symbol from helpers.js...

We can now use UI.registerHelper. And since we're calling api.use('ui') we don't
need to check if UI or Handlebars is defined. Just call UI.registerHelper.
Bump blaze-layout dep version to 0.2.4.
Make dataNotFoundHook return if data not ready.
Only call the data function if the controller is ready.

cc @tmeasday.

This solution creates a reactive dependency on this.ready() inside the data set
computation. But if the controller is not ready, we still set the global data
context to null. This prevents previous data contexts from sticking around.
Actually put the data() call inside the ready if block!
Make ui dependency required, not weak.
Bump version to 0.7.1.