Various db upgrades

[kindly]

Three database upgrades bundles together in single pull request for convenience.

• Metadata modifed

Make metadata modified a field in package table to make it possible for @wardi to speed up package show

• creator user id

cache datasets creators user id

• url type

Add url type for later making relative resource possible

[kindly]

@wardi would be great if you could help review this one.

[wardi]

This looks like what I need.

Is there some documentation for the intended use of url_type? I wonder if
it's similar to a resource extra we're already using.

[kindly]

@wardi

There is no documentation yet and is a placeholder for future needs as we have two uses for it.

• saying the resource is held in the local filestore.
• saying the resource is one held in the datastore.

This means that we can put in the url field not just full url paths but relative urls or anything relevant to the particular url type. (the datastore one for example will point to a way to get a csv copy of the data)