[theme] refactor src theme build

This PR refactors how the theme src of this fork is managed. Now the changes of this fork compared to searxng/searxng upstream are tracked in definitions.less and search.less. The update.sh, which is used to build the prod CSS files now does not depend on docker anymore. It rather clones the latest upstream searxng/searxng repo, copys the changed files of this fork into the right place and executes the upstream build script. After that it copys the build files back into the src dir of this fork. This means docker is no longer needed for dev, but now python, npm and make are required. (This also enabled easy development execs with the upstream scripts in the future...)