keep favorite/star button's title in sync with starred status

Meta: [‘Favourite’ Should Read ‘Unfavourite’ When a Topic Is Favourited](http://meta.discourse.org/t/favourite-should-read-unfavourite-when-a-topic-is-favourited/3231) & [Filtering topic by user is very buggy/laggy](http://meta.discourse.org/t/filtering-topic-by-user-is-very-buggy-laggy/4813/3)

This will keep the title of both the favorite button and the stars in sync with the starred status of the topic.

![Screenshot_18_03_13_00_22](https://f.cloud.github.com/assets/362783/268801/85cc6d32-8f59-11e2-83b6-327da3c71bb6.png)

- [x] added **2** methods to the `topic` model
 - `favoriteTooltipKey` that will return the translation key depending on the *starred* status of the topic
 - `favoriteTooltip` that will return the translated string based on the previous method
- [x] binded the title attributes to `favoriteTooltip` for the stars (in the `topic_list_item`, `topic` and `topic_extra_info` templates)
- [x] binded the `helpKey` property to `favoriteTooltipKey` for the favorite button
- [x] added globalization for the **2** strings

I've checked the following places:
- the star on a topic list
- the star next to the topic title when viewing a topic
- the star next to the topic title when viewing a topic and scrolled down
- the favorite button when viewing a topic

Please, do not hesitate to tell me whether I missed one :wink: