Allow user to hide the message to add an ssh key permanently

This implements the feature in http://feedback.gitlab.com/forums/176466-general/suggestions/4912594-allow-user-to-hide-the-message-to-add-an-ssh-key-p

In this particular implementation the user is given the choice between permanently dismissing the message or just dismissing it for the current browser session. I believe there is value in retaining both options.

Also, I opted to display the two choices as links instead of buttons. I initially favored the buttons and they look very nice. However, I think they attract too much attention. I sought opinion on IRC and several others came to the same conclusion.  I am open to either solution.

Current solution:
![screen shot 2013-12-14 at 8 18 26 pm](https://f.cloud.github.com/assets/2394772/1749667/fff80dbc-652f-11e3-8f7e-7dba98fdfcd0.png)

Alternate solution with buttons:
![screen shot 2013-12-14 at 7 50 08 pm](https://f.cloud.github.com/assets/2394772/1749669/07c69ea0-6530-11e3-9fdc-8d4d7e4ae6dc.png)