Implement sticky date separators

Use `react-sticky` to implement sticky date separators. This will pin a date separator to the top of the timeline panel when the separator scrolls out of the top of the view.

A known issue of this is that the spinner, which is in line with event tiles in the timeline, will appear to push the stuck date separator down. In reality the first date separator after the spinner is in line with event tiles and is not stuck because the spinner forces the timeline to be scrolled slightly further down than it would be otherwise. But also, date separators in the timeline (not "stuck") have a greater height.

Ideally the date separator would be suppressed whilst back paginating, but this will cause the stuck separator to flicker on and off. This is why the suppression has been removed.

src/components/structures/TimelinePanel.js

@@ -1125,7 +1125,6 @@ var TimelinePanel = React.createClass({
                           highlightedEventId={ this.props.highlightedEventId }
                           readMarkerEventId={ this.state.readMarkerEventId }
                           readMarkerVisible={ this.state.readMarkerVisible }
-                          suppressFirstDateSeparator={ this.state.canBackPaginate }
                           showUrlPreview = { this.props.showUrlPreview }
                           manageReadReceipts = { this.props.manageReadReceipts }
                           ourUserId={ MatrixClientPeg.get().credentials.userId }