Finalize 2.0.0

[zonky2]

TODO:

• Bump PHP and Contao version (see change all composer.json: required PHP to 5.4 and contao/core to 3.5 core#1011)
• Fix all remaining tickets tagged for 2.0.0
• Check all file header (like https://github.com/MetaModels/core/blob/master/src/MetaModels/MetaModel.php#L1-L26)
• Add .check-author.yml or check this
• Add .github/ templates - see example but ensure to update the links in the files!
• Add .gittattributes file (see Add .gitattributes files core#846)
• remove minimum-stability from composer.json and add "metamodels/core": "~2.0@alpha" and "contao-community-alliance/dc-general": "~2.0@beta"
• Ensure LICENSE file is present! (see https://github.com/MetaModels/core/blob/master/LICENSE)
• Update badges in README.md! (look at first rows https://raw.githubusercontent.com/MetaModels/core/master/README.md)
• Check build status is green!