Provide offline capabilities to tasks page

[jaragunde]

With new web standards like service workers, we should be able to implement the tasks page in a way that could work offline, letting users edit their tasks in any moment and synchronizing with the server whenever connectivity is available again.

This change will probably require to make the tasks page behave like a single page application, like specified in #421. It will also increase the complexity of the potential conflict checks, we will have to control situations we currently ignore like, for example, how tasks created from a different browser tab (or browser, or device) affect to previously opened sessions.