Update install tusk.sql to source db files

The tusk.sql file used in the install process dropped databases
and loaded everything. This functionality was modified in
db/baseline.pl to not drop databases in existing installations.
tusk.sql now loads the mysql baseline files from the db
directory to avoid duplicating data and efforts.

This change makes no impact on current installation instructions;
they will work on new installs as always.

When adding to the databases, from now on developers should
follow the database versioning and upgrade guidelines
available at:
https://wikis.uit.tufts.edu/confluence/display/TUSK/TUSK+Database+Versioning