db/schema/openquake.sql:

Updates to FK constraints so that deletions of `*_calculation` records are
cascaded. This makes deleting these sets of data extremely easy and
straightforward.
bin/openquake:

Added new command line options for deleting risk and hazard calculations.
engine2:

Added a util function for getting the current OqUser.
engine2:

Added del_risk_calc and del_haz_calc functions, which include handling for
cases where deletions are not allowed.
bin/openquake:

--dhc and --drc options are now wired up to the corresponding functions in the
engine code.
tests/utils/helpers:

Removed the implicit assumption in `get_risk_job` of job config files
always being in the demos/ folder.

This is also more consistent with `get_hazard_job`.
tests:

Updated usage of `get_risk_job` to fix broken tests.
tests/utils/helpers:

Updated docstring of `get_risk_job`.
tests/engine2_test:

WIP for testing hazard/risk calculation deletion cases.
db/schema/security.sql:

`oq_admin` has full SELECT/INSERT/UPDATE/DELETE access to all tables (not just
delete).
engine2:

Correct a minor semantic error in del_haz_calc.
engine2:

Added a couple of comments to del_haz_calc.
tests/engine2_test:

Added tests for del_haz_calc and del_risk_calc. These functions now have 100%
coverage.
Merge pull request #1068 from larsbutler/delete-calculations

New CLI functionality: delete old calculations

[r=matley] [f=1117052]