[DOC] Add docs for MutableArray addObject and removeObject

* Add docs for addObject & remove Object, closes #4618
* Clean up code style and various docs on MutableArray
* Update old observer syntax example
Merge pull request #4624 from mixonic/doc-add-remove-object

[DOC] Doc add remove object
[FEATURE computed-read-only] Remove feature flags.
[FEATURE ember-metal-is-blank] Remove feature flags.
[FEATURE ember-eager-url-update] Remove feature flags.
[FEATURE ember-routing-auto-location] Remove feature flags.
[FEATURE ember-routing-bound-action-name] Remove feature flags.
[FEATURE ember-routing-inherits-parent-model] Remove feature flags.
Merge pull request #4623 from rjackson/remove-released-flags

Remove feature flags for 1.5.0 enabled features.
Update CHANGELOG.md for 1.6.0-beta.1.
Post release version bump.
Remove dup changelog entry.
Merge pull request #4638 from ugisozols/dup_changelog

Remove dup changelog entry.
[DOC release] Clarify reduceComputed's docs

I felt it was unclear what "completely recomputed" meant in the context of non-array or `.[]` (square bracket) dependencies. Hopefully this should help others understand more quickly.
Merge pull request #4642 from Kerrick/patch-1

[DOC release] Clarify reduceComputed's docs
Merge pull request #4516 from bradfol/cleanup-linkto

Cleanup unused var
[BUGFIX beta] incrementProperty should parseFloat before incrementing
Merge pull request #4249 from oskarols/render-viewname-fix

[BUGFIX #4248] Route#viewName overriding explicit name parameter when calling render method
[BUGFIX beta] Drop dead code for * in paths
Add count to array computed meta