Embed plugin: use empty embed_provider by default

[Wiktor Walc]

The embed plugin has now //ckeditor.iframe.ly/api/oembed?url={url}&callback={callback} URL set by default as providerUrl.

As the default anonymized API end point has certain restrictions on its usage, it's better to make sure that each user using this plugin is aware of limitations (otherwise there is a risk that suddenly on a production website this service will stop working_. This effect can be only reached by removing the predefined end point n the plugin configuration and having users reading the documentation in order to specify it manually.

The documentation should list //ckeditor.iframe.ly/api/oembed?url={url}&callback={callback} as a "ready to use" option, but it should also recommend setting up an account on Iframely to better track the API usage (even in the free "Developer" tier plan).

[Tade0]

Updated the default and the docs accordingly, changes pushed to ​branch:t/13793.

Relevant pull request in the docs repo: ​https://github.com/ckeditor/ckeditor-docs/pull/71

[Marek Lewandowski]

Editor should report an error explaining the reason of the problem in case of missing embed_provider. User ​`error` method for this purpose.

[Tade0]

Added this. Error explained in the associated docs pull request.

[Marek Lewandowski]

Pushed a couple of changes to t/13793 branch, now it looks pretty much fine, but the related docs pull request needs to be adjusted.

[Marek Lewandowski]

Moved to major milestone, as we don't want to make such a breaking change in a minor release.

[Tade0]

Updated the docs as requested.

[kkrzton]

Rebased on current major, corrected docs typo and added unit tests. Created PR ​https://github.com/ckeditor/ckeditor-dev/pull/342.

[Marek Lewandowski]

Fixed with ​git:9afd0df0c278280d5471cd333ec696aac57db384, merged to major.