Add build weather support.

* src/cuirass/database.scm (build-weather): New macro.
(build-status->weather): New procedure.
(db-get-builds): Return the build weather using the new procedure.
* src/cuirass/http.scm (build->hydra-build): Also return the weather.
* src/cuirass/templates.scm (weather-class, weather-title): New procedures.
(build-eval-table): Display the weather.
* tests/database.scm ("db-get-build weather"): New tests.
* tests/http.scm (build-query-result): Adapt it.