This commit adds a migration that creates a view of the HTTP headers in the response table. Once the view is in place you can run a query like this without requiring JSON parsing:

```sql
SELECT warc_record_id, name, value FROM http_headers;
```

It can be helpful for identifying for things like:

```sql
SELECT
  value,
  COUNT(*) AS count
FROM http_header
WHERE name = 'content-type'
GROUP BY value
ORDER BY count DESC;

value                              count
---------------------------------  -----
application/javascript             57
image/png                          11
text/css                           7
text/html; charset=utf-8           6
image/jpeg                         4
image/gif                          4
text/fragment+html; charset=utf-8  3
image/svg+xml                      3
text/plain                         2
text/html; charset=UTF-8           1
```

Closes Florents-Tselai#24

Add a similar table for HTTP requests. Prefix the view names with a `v_` to distinguish it in the schema from
actual tables.

Also add a description of the view with a table that defines the columns.