Deprecate before observers

Also remove usages of public `addBeforeObserver` in ember internals.
[BUGFIX] Allow Ember.Select to have selection from a Promise
when multiple=false
Helpers return content instead of updating a morph
Merge pull request #9690 from mmun/helpers-return-content

Helpers return content instead of updating a morph
Ensure that `{{unbound if foo}}` works properly.
Merge pull request #9687 from rwjblue/more-unbound-fixes

Ensure that `{{unbound if foo}}` works properly.
Ensure `Ember.Handlebars.makeViewHelper` is available.
Merge pull request #9691 from rwjblue/htmlbars-makeViewHelper-compat

Ensure `Ember.Handlebars.makeViewHelper` is available.
Updates HTMLBars version
Merge pull request #9692 from twokul/htmlbars-comments

Updates HTMLBars version
[DOC] Fix `Ember.copy` Documentation

The documentation was incorrect: one must implement `copy()`, not
`clone()`.
Merge pull request #9695 from jherdman/em-copy-docs

[DOC] Fix `Ember.copy` Documentation
Use `appendView`
Merge pull request #9696 from twokul/view-tests

Use `appendView`
Ensure HTMLBars files are labeled for the correct module.
Merge pull request #9698 from rwjblue/htmlbars-docs-modules

Ensure HTMLBars files are labeled for the correct module.
Add deprecation for old usages of cachable and readOnly in CPs

* Passing opts.cacheable to the constructor of ComputedProperty is deprecated
* Passing opts.readOnly to the constructor of ComputedProperty is deprecated
* Invoking `cacheable` in a computed property is preprecated.
* Invoking `readOnly` with aany params in a computed property is deprecated.
* Make InjectedProperty inherit `readOnly` from CPs prototype
Update out-of-date JSDoc; rename parameter '_path'
Moves `{{view}}` tests
[Bugfix beta] this isn’t observable, and turns out its costly.