Make more fields non-null

[jyn514]

Part of #874.

I ran the following queries in prod with 0 results:

select count(*) from queue where name is null;
select count(*) from queue where version is null;
select count(*) from queue where priority is null;
select count(*) from queue where attempt is null;
select count(*) from doc_coverage where release_id is null;
select count(*) from releases where version is null;
select count(*) from releases where rustdoc_status is null;
select count(*) from releases where build_status is null;
select count(*) from releases where have_examples is null;
select count(*) from releases where is_library is null;
select count(*) from authors where name is null;
select count(*) from owners where avatar is null;
select count(*) from owners where name is null;
select count(*) from crates where github_stars is null;

r? @Nemo157