Content translation/V2

Content translation version 2 is a revamp of the translation system with OOJS based code organization and OOUI based front end. This will also have the editor component enhanced using Visual Editor.

The plan is to gradually replace version 1 with version 2. Version 2 won't be an incremental update to version 1. But the translations created using version 1 should work with version 2 too.

Rationale
CX was developed iteratively for last 2+ years. During that time, features were added in several stage of product development. In the beginning of the project, there was a highlevel understanding of features but not the details. So the architecture was a flexible one where modules can be plugged and get this done. This worked very well for us and we were able to add features fast. But the approach caused code duplication, less organized hierarchy of similar usecases. So a consolidation and reorganization of these use cases is much needed.

The general direction in which CX is moving is to align with the MediaWiki extension good practices which includes OOUI based standardised frondend. Also CX need much improved editing experience with the help of VE. OOJS and obvioulsly VE follow object oriented architecture heavily. CX's plugin based and global hooks based architecture is ill suited to mix with them. So the above mentioned organization should follow OO architecture for the best match.

The patched up CX code showed some shortcomings recently and we can solve it during this reorganization.

Major features

 * OOUI based translation interface
 * Visual Editor as editor
 * Machine translation support for Template params, reference texts and practically all kind of elements in screen. In version 1, machine translation was limited to paragraphs alone
 * Fixing lot of bugs that was too difficult to handle with previous version
 * More editing tools, inherited from VE, such as table editing, link editing
 * Better support for References and Templates
 * Ability to add and remove categories
 * UI revamp based on UI Standardisation initiative

Status
The new version is always available in production and can be accessed using version=2 in the normal CX translation view. At present this is highly unstable.The progress is tracked using

Rollout plan
A rough plan is to enable version 2 is smaller wikis or subset of wikis to do QA and gradually rollout to more wikis.

Backwards compatibility
Versions 1 and 2 will coexist during a transition period. Given that the translations each version produce are not expected to be compatible, the following steps are considered to avoid issues related to breaking backwards compatibility:
 * 1) Translations started with one version of the editor will be alway opened with the same version, regardless of which is the current default editor. That is, when version 2 is the default, old translations started with the version1 will still be opened with version 1.
 * 2) Once version 2 is considered the stable default, creating new translations with older versions will be prevented. That is, version 1 will not be available to create new translations, but it will be still available to edit the old ones.
 * 3) With a process in place to automatically discard translations after one year, version 1 could be safely removed after such period pases since no new articles can be started with it.