Add report mailer preferences editor

[preaction]

Once we can validate PAUSE accounts (#14), we can allow users to edit their mailer preferences. See cpan-testers/cpantesters-schema#32 for the details of the data. Each author can set preferences for any of their distributions, and a default for any distribution they don't configure individually.

The editor should show the default settings and each distribution as rows in a table. Clicking an "Edit" button next to the row should show a form to adjust the settings. There should be a "Delete" button in the row to delete an individual distribution's settings to return them to the default (users can not delete the default settings). There should be an "Add" button above the table to create a new preferences entry for a distribution. Authors are not required to have already released a distribution to add preferences for it (so they can configure a distribution before they make a release), but we should warn them if they try to configure a distribution that we've never seen before (in case they mistype the distribution name).