Define and implement pagination

[britram]

Certain resources should allow pagination, with next/prev links:

• GET /raw: list campaigns probably not useful
• GET /raw/<campaign>: list files in campaign
• GET /obs: list observation sets
• GET /query: list cached queries see query cache should allow paginated listing by type #25
• GET /query/<qid>/results rows done

[britram]

this is done for /query/qid/result.

listing the query cache needs to be rethought a bit anyway, so will defer to a new issue.

the natural ordering of obsets is by ID. the natural ordering of files in campaigns is lexical by name. The former might need to be pushed back into the database eventually, but we can paginate in RAM for now.

It's not clear this is useful for listing campaigns, so will defer this.

[britram]

Done for this release.