Fix sections.migrated_time

[notartom]

Next on the STRICT_TRANS_TABLES fallout is sections.migrated_time. It's preventing us from creating a new project from a project code because there is no default value.

We're not really sure what sections.migrated_time is for or where it came from. It looks to be intended as a duration, but it's filled with invalid values and other garbage. Its only reference in the code is in the RSS view application/views/rss/rss.php. We want to give it a default value of NULL, and in the process, let's clean it up and make it a proper TIME column.

alter table sections modify column migrated_time varchar(20) null;
update sections set migrated_time = NULL where migrated_time = 'Source:';
update sections set migrated_time = substring(migrated_time,1,8) where migrated_time like "%Source:";
update sections set migrated_time = NULL where length(migrated_time) > 8;
update sections set migrated_time = NULL where migrated_time like '%Sung%';
update sections set migrated_time = NULL where migrated_time = '';
update sections set migrated_time = NULL where migrated_time like '%by%';
update sections set migrated_time = '00:15:07' where migrated_time = '00:15"07';
update sections set migrated_time = '01:13:50' where id = 126048;
update sections set migrated_time = NULL where id = 163744;
update sections set migrated_time = NULL where migrated_time like '1599%';
update sections set migrated_time = NULL where migrated_time like 'Book%';
alter table sections modify column migrated_time time null;