Refactor dates and times in the Commits table

> [!NOTE]
> Migrated from [augurlabs/augur#1514](https://github.com/augurlabs/augur/issues/1514)
> Originally opened by `@jberkus` on 2021-11-13

---

(1) The dates in augur_data.commits have been stored in character varying fields. This forces coversion to and from date types every time they're used, and could lead to code errors:

Fields in question:

```
cmt_author_date │ character varying(10)
cmt_committer_date │ character varying
```

(2) Of the other datetimes in the table, half are timestamp with time zone, and half are timestamp without time zone. These are certainly already leading to query errors:

```
cmt_date_attempted │ timestamp(0) without time zone
cmt_ght_committed_at │ timestamp(0) without time zone
cmt_committer_timestamp │ timestamp(0) with time zone
cmt_author_timestamp │ timestamp(0) with time zone
data_collection_date │ timestamp(0) without time zone
```

Was there a reason this was done, or was it completely a design error?