Show placeholder tweets if we don't have the tweet data client-side yet

[keithamoss]

Right now we rely entirely on loadMoreRows from react-virtualized to handle loading tweets. React-Virtualized has the option to show "placeholders" - for example, as the user is scrolling or as loadMoreRows is loading data. As we move towards issues like #26 we'll need to potentially sync a large number of tweets in.

Let's investigate this further and see how we can use it to improve the UX of seeing at least something for the tweet placeholders rather than blank spaces.We could avoid the need to have all tweet objects client-side if we instead showed placeholder tweets in the columns as the user is scrolling (up or down), fire events up to the store, debounce/wait those events OR wait until the user stops scrolling, then fetch them as a batch, shove them into the store, and replace the placeholder tweets with the actual tweets (and recalculate the cell heights).

This would handily remove issues around having to send potentially huge payloads when the user is scrolled a long way down the column and we're trying to send them all tweets between now and their current position.

Relates to #26.