Feature request #1676: Rename "default" column on Email model

As DEFAULT is a reserved keyword in MySQL, working with this column may require special quoting; eg:

ALTER TABLE emails MODIFY default BOOL DEFAULT 1;

won't work; it must instead be written:

ALTER TABLE emails MODIFY `default` BOOL DEFAULT 1;

Rename the column to avoid any potential confusion.

Possibly suitable synonyms include preferred, principal, perhaps active, standard, preset...