Hide application behind authentication

[bhermann]

The management application should not be available to the public, but only to administrative users.
Therefore, the first thing a user should see is user authentication. There should NOT be any public information available besides the version of the application.
Of course, we are not reinventing the wheel here so this issue is a task about configuration and the introduction of a fitting component. Also, for this issue it is okay to use a fixed user account until we get user account management (#12).

For this, have a look at this list from the Play website: https://www.playframework.com/documentation/2.6.x/ModuleDirectory#Authentication-(Login-&-Registration)-and-Authorization-(Restricted-Access)
On a first look Silhoutte seems pretty professional, but I might be wrong.

Keep in might that we would like to have a user account management (#12) and two-factor authentication (#1) at some point in the future. It should be possible with the choice made here.

[bhermann]

Closed with #20.
Thanks @Ayybeeshafi and @johannesduesing.