Fix Asset precompilation

[MrSerth]

Up to now, all assets machine the pattern '*.js', '*.css', '*.css.erb', '*.coffee', '*.scss' were precompiled. That yielded an error during deployment after merging #230:

Tasks: TOP => deploy:assets:precompile
(See full trace by running task with --trace)
The deploy has failed with an error: Exception while executing as codeharbor@codeharbor.xi.xopic.de: rake exit status: 1
rake stdout: Nothing written
rake stderr: rake aborted!
SassC::SyntaxError: Error: Undefined variable: "$navbar-height".
        on line 16:16 of ../../shared/bundle/ruby/2.6.0/gems/rails_admin-2.0.1/app/assets/stylesheets/rails_admin/base/theming.scss
>>   padding-top: $navbar-height;

   ---------------^

This is related to this issue in rails_admin suggesting not to precompile all assets.

Hence, I disabled that behavior with f378a18 and tested the result in the production environment locally without error. Nevertheless, we should evaluate whether no other assets are required and update production.rb accordingly.

[kkoehn]

The only thing that does not seem to work on a quick walk through locally is ACE, I'll double check and try to fix it