tt-rss: Enable single sign on for tt-rss

Created by: SunilMohanAdapa

Complexity: 2 points

Currently, when a user is logged into Plinth, they are not automatically logged into tt-rss application. When tt-rss application is opened, they are asked for another login. Eliminate this extra login by using the new single sign on framework.

• Assumption: Single sign on framework is already available. See #859.
• This should use the same pubtkt framework as the rest of the Plinth.