Changes

Summary

1. read prefs properly;
2. allow explicit Isabelle rev in build task (e.g., for older Isabelle versions);
3. web app: proper document height;
4. merged
5. replace manual def. of timing function
6. add build manager module;
7. support ci job via hg_sync (cf. 7883f221d6d3);
8. tuned;
9. use Content-Digest header in HEAD requests instead of length (to track non-monotone changes);