Require author and translator dates are numeric

This goes some way to addressing issue LibriVox#154 -- users won't be able to
enter non-numeric dates, which is unfortunate, but it does mean that
they'll get a proper error message instead of the random server error
that they get right now.

For the more technical explanation -- When we save the form generation,
that all goes into the `form_generators` table. Any new authors that are
created as part of the generator are saved into
`form_generators_authors`, which has the `auth_yob` and `auth_yod`
columns as four-digit integers. This doesn't align well with the actual
`authors` table, which allows pretty much anything (`varchar(10)`), and
I have no idea how people specify AD vs. BCE using the generator.

I don't know enough about how the table is used to know whether we
should consider migrating it to `varchar(10)` to match `authors`, but
having the two consistent with each other would be good probably.

I also tried specifying the input fields as numeric in the HTML, but
that didn't quite have the desired effect -- it would just submit the
field as empty if it wasn't right, which meant we'd silently drop the
data rather than get a validation error.

application/controllers/public/Project_launch.php

@@ -209,15 +209,15 @@ private function _validate_form()
 		$this->form_validation->set_rules('auth_id[]', 'Author id', 'trim|xss_clean');
 		$this->form_validation->set_rules('auth_first_name[]', 'lang:proj_launch_auth_first_name', 'trim|xss_clean');
 		$this->form_validation->set_rules('auth_last_name[]', 'lang:proj_launch_auth_last_name', 'trim|xss_clean');
-		$this->form_validation->set_rules('auth_yob[]', 'lang:proj_launch_auth_dob', 'trim|xss_clean');
-		$this->form_validation->set_rules('auth_yod[]', 'lang:proj_launch_auth_dod', 'trim|xss_clean');
+		$this->form_validation->set_rules('auth_yob[]', 'lang:proj_launch_auth_dob', 'trim|xss_clean|numeric');
+		$this->form_validation->set_rules('auth_yod[]', 'lang:proj_launch_auth_dod', 'trim|xss_clean|numeric');
 		$this->form_validation->set_rules('link_to_auth[]', 'lang:proj_launch_link_to_auth', 'trim|xss_clean|prep_url');
 
 		$this->form_validation->set_rules('trans_id[]', 'Translator id', 'trim|xss_clean|numeric');
 		$this->form_validation->set_rules('trans_first_name[]', 'lang:proj_launch_auth_first_name', 'trim|xss_clean');
 		$this->form_validation->set_rules('trans_last_name[]', 'lang:proj_launch_auth_last_name', 'trim|xss_clean');
-		$this->form_validation->set_rules('trans_yob[]', 'lang:proj_launch_trans_dob', 'trim|xss_clean');
-		$this->form_validation->set_rules('trans_yod[]', 'lang:proj_launch_trans_dod', 'trim|xss_clean');
+		$this->form_validation->set_rules('trans_yob[]', 'lang:proj_launch_trans_dob', 'trim|xss_clean|numeric');
+		$this->form_validation->set_rules('trans_yod[]', 'lang:proj_launch_trans_dod', 'trim|xss_clean|numeric');
 		$this->form_validation->set_rules('link_to_trans[]', 'lang:proj_launch_link_to_trans', 'trim|xss_clean');
 
 		$this->form_validation->set_rules('project_type', 'lang:proj_launch_type_of_project', 'trim|xss_clean|alpha_dash');