Get adminer via composer

[vmcj]

We should composer require and run this in the post install / update cmd.

[meisterT]

Can you please clarify what "run this" means in this context and why we don't already use composer?

[vmcj]

A solution would be to:

• composer require adminer/adminer
• compile the different adminer files to a single file in the composer post-install-cmd
• Store that file in the right location and add our changes somewhere else.