Add cursor-based pagination

[rafalp]

This PR adds misago.core.cursorpaginator.CursorPaginator utility for implementing cursor pagination in Misago.

Fixes #1066

TODO

• Create pagination util
• Move threads lists to cursor pagination
• Remove MISAGO_THREADS_TAIL setting (from settings.py and docs)
• Move user posts/threads/username history lists to cursor pagination

[rafalp]

Observation after quick tests:

On larger dataset (130k threads) cursor based pagination is 20x faster than offset pagination, returning results in 5ms vs 100ms.

[rafalp]

Note: for now I am skipping the search, because its ordering the results by accuracy. I'll revisit it down the road.