Bunch of changes solving aging problem when arrival field not available, and other things

This is in fact a bunch of changes:

* The only one affecting the vizgrimoire directory should be the modification of ages.py to use the new "check" option for MLSActivityPersons, to check if arrival date fields are non-null for all records in the messages table. If that's not the case, it uses first dates.

* Support for the above in grimoire_alch directory.

* Some reordering in grimoire_alch directory, wtih a new reports directory, for reports using libraries in this directory.

* Some support for code review (using the Gerrit backend for Bicho). Still very preliminary.

* Some implementation (very preliminary) of Polarsys maturity model metrics.

* Some fixes and improvements here and there.

I've checked the grimoire_alch part, and it should not affect the vizgrimoire part (except that now MLS databases without arrival records should work for aging).