Added: search

* I created a SearchStore to maintain the app state of it
* I abstracted a bit the message-list component so it can handle search result as well as messages from a mailbox
* I cheated a bit for the pagination management (it's done server-side) because the indexer doesn't send the number of results (yet). So page numbers and number of results will be incoherent but it works fine (it'll paginate until there is no result left to show)
* I added a new function to the RouterMixin to ease redirections
* For testing purpose, I added a route '/messages/index/' that will index all the messages