[www] Add support for pagination to the proposal list routes

[sndurkin]

Pagination should be done like reddit's API, with support for the following:

• after: the id of the last proposal on the previous page
• before: used for navigating backwards through the list in the UI, it's the id of the first proposal on the previous page
• limit: (optional) the number of proposals to fetch in the page

[go1dfish]

We need to be able to paginate api endpoints for proposal lists.

I recommend the approach reddit uses.

A list is sorted in some way (let's assume new)

To get a page, I specify the limit (max number of items returned) and an optional start/end point.

You could have a default limit of 20.

For these examples, assume the list is sorted by newest first.

To get a page of i items I fetch

/vetted?limit=20

To get the next oldest page, I would fetch:

/vetted?limit=20&before=<idofthelastiteminpreviouspage>

A meta key can be added to the response as a sibling to proposals

{
  proposals: [...]
  meta: { total: 9123 }
}