Add "primary" class and "log-in" Glyphicon to "Log in" button

@markcarver

Before I posted the proposal, first I thought "primary" class. In theory, yes, It seemed to be fine. However I prefer "success", which is green because I feel "primary" class, which is blue, is lower profile since the default URL link color is blue. In Bootstrap, a submit button is "success" class, which is green. The user experience starts with login button, therefore I thought "success" (green) is even more consistent and suitable for this login button. I wanted to respect such UX.

There looks no rule to apply "primary" or "success" to login for the a web site using Bootstrap. I searched google image with a keyword "login glyphicon", they are voting half and half before I proposed the first patch. That is, some sites employ "primary" that is "blue" for login button, on the other hand, some sites "success" that is "green". I checked several major sites such as Facebook, Twitter, Google, LinkedIn and d.o. and found they apply the site's color scheme. Especially the login button color of d.o. is green, but it looks naturally matching the color scheme of d.o. Note that d.o. employ "green" for buttons, and "blue" for links.

Attached screenshots are my site's login screen with "primary" or "success". Hope that helps.

@markcarver,

Understood. And let me post the backport patch to backport to 7.x-3.x.

Oh, sorry for my mistake. Hope this works.