Renaming Chair to Employer

This pull request would merge the changes we discussed for the chair model. It got renamed to Employer, in addition a boolean "external"-flag was added to indicate external employers and also the head_of_chair field was renamed to head. This merge contains also all the necessary changes resulting from the mentioned renaming. All tests were adapted and are passing, the translations got updated accordingly and also a manual check for maintained functionality was done. Finally the project was searched using a RegEx for ".*chair.*" and does not contain any more matches except for previous migrations.