Only show sign in box in PSF pages.

[berkerpeksag]

Fixes #536.

[Mariatta]

I think this works and definitely agree that the sign in box should only appear where it needs to be.

My only concern is that this creates inconsistency with the placement of the search textbox.
In most pages the search box will be more to the right of the page, whereas in the PSF page, where the sign in page is shown, it will be slightly to the left.

Perhaps not a big deal though, but then again maybe not my call.
Is there anyone who can provide input on this?

Thanks.

[berkerpeksag]

With this PR applied, screenshot of the homepage:

Screenshoot of the PSF landing page:

[MarkMangoba]

@berkerpeksag amazing looks so much cleaner.

[berkerpeksag]

I noticed a bug. I think we should still show "Your account" link in every page if user is already logged in:

I've fixed it in the latest version of the PR.