Infinite scrolling

[rjwebb]

This PR adds a simple mechanism for infinite scrolling to the Polls and FIP Tracker pages.

Since all of the conversation data is loaded onto the client, the point of this feature is not to reduce load on the database but purely to make the UI more responsive - if we are trying to display all of the conversations and apply a filter, this can involve having to reflow the layout for hundreds of components, which can take hundreds of milliseconds, leading to noticeable lag. It seems like the problem here really is just reflowing the layout, I had hoped that it was something to do with React memoising props since that would have been fixable by refactoring the components themselves...

Changes:

• A (number, integer) state scrollCursor which shows the current "cursor"
• An invisible IsVisibleObserver component which is added to the end of the list of items. When it comes into view, increment scrollCursor by some fixed amount (e.g. 10)
• When we display the conversations, only show the first scrollCursor + pageSize. Set pageSize to whatever the number of items to initially show is.

Note that this mechanism assumes that the filters can only be set from the top of the page - which is currently how the filter dropdowns work.

[raykyri]

looks great