Add method wp.api.getModelByType for retrieving api models

[mkaz]

For Gutenberg, it will be easier and less error prone to fetch the wp.api.model by retrieving by type instead of creating a route and searching for the model that matches that route.

An open detail to work out is if namespacing is needed, can objects have the same name in different API namespaces in the same instance of wp.api. A potential solution could be an optional second argument.

For example: getModelByType( type, versionString = 'wp/v2' )

Related to #41111

Original Slack Discussion at:
​https://wordpress.slack.com/archives/C5UNMSU4R/p1511878493000062

[mkaz]

The reference implementation is available at PR #3683 on Gutenberg
​https://github.com/WordPress/gutenberg/pull/3683

[mkaz]

Patch attached.

The mapping associative arrays were created outside of the getModelByType and getCollectionByType functions so that they can be extended or overwritten if necessary.

[mkaz]

This is no longer needed to support Gutenberg, the calls were changed to no longer map into Backbone models but to go straight to the API for data.

Change made in ​https://github.com/WordPress/gutenberg/pull/5253

I still think this change is better way to map models instead of the routes which can lead to error, but probably doesn't make sense to include if there isn't going to be any code to use the new functions.

[danielbachhuber]

Bumping out of 5.0 because it's not necessary for Gutenberg.