Add tooltip for "Connect Now" button

[davinotdavid]

When we click the "Connect Now" button, we don't know if the user has Thunderbird Desktop installed or not so we should proactively show a helpful tool-tip with instructions to download and install it if need be.

[thundernotion]

Issues cannot be both on the Milestones and Tasks boards. This appears to be a Task, removing from Milestones board