Added caching to alert fragments related to the read view.
WIP: add fragment caching to the sidebar.
Added caching to sidebar.

Several fragments involved in creating the sidebar have no ruby code, not even i18n translations, they are plain HTML. Those fragments are not cached, there would be no improvement if we did.
Added fragment caching to the start page and associated fragments.
Added fragment caching to entries list and read view.

These are angularjs views, which really take the aspect displayed to the user when the angularjs client initializes. The HTML served is the same for all users (locale aside), so a high cache hit rate is expected.