Finalize 2.0.0

[zonky2]

TODO:

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

[discordier]

Depends on #9. @zonky2 please test this.

[zonky2]

RTM