Make the logo configurable via hydra.conf

Dependencies

• [2] XJRJ4J7M Add user registration
• [3] MGOGOKQP add tracker html code via HYDRA_TRACKER
• [4] 6KJXJB7N qualify ordery_by clauses when necessary, remove unnecessary order_by's, reported by Ludo, resulted in errors in sqlite
• [5] HRAFVVOE make logo configurable via HYDRA_LOGO env var
• [6] IZEXRZWT Replace old logo with Nix/NixOS logo.
• [7] EFWN7JBV * Added a status page that shows all the currently executing build steps.
• [8] G4X5IUYJ Remove default logo, replaced by text for now. Hide template in jobset edit.
• [*] J5UVLXOK * Start of a basic Catalyst web interface.

Change contents

• replacement in src/lib/Hydra/Controller/Root.pm at line 23
  [3.134]→[3.21:80](∅→∅)
  − $c->stash->{logo} = $ENV{"HYDRA_LOGO"} ? "/logo" : "";
  [3.134]

  [2.40]
  + $c->stash->{logo} = $c->config->{hydra_logo} // ($ENV{"HYDRA_LOGO"} ? "/logo" : "");