Yii 2.2 Release

[rhertogh]

Now that Yii 2.0.50 has been released (https://www.yiiframework.com/news/639/yii-2-0-50) I think it would be good to focus on the release of Yii 2.2.

As discussed here Yii 2.0.x will be "bug fixes and security fixes only" from now on, so new features should target the 2.2 branch.

@terabytesoftw Since you have been the main driver behind the 2.2 branch, which tasks remain before 2.2 can be released? Input from other @yiisoft/core-developers is of course greatly appreciated.

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Tasks (full list yet to be determined):

• [ ] Create a 2.0 branch (which can be used for bug and security fixes)
• [ ] Merge the 2.2 branch into master
• [ ] Update release info on website and readme.md