Jetpack Plugins: Should be able to update all plugins from the 'Updates' tab with one click

[v18]

Reported by @enejb: The plugins update tab looks like this

Can we make it easier from this tab to update all the plugins?
I think it is safe to assume that when a user is on this tab that they want to update plugins.

The way to do that now is:

1. to click into each of the plugins
2. click the update button

or

1. click manage
2. select all plugins
3. click update.

Here's a mockup of what this can look like:

It's related to #912 - adding a manage bar section