Translations I

Closes #305 
Continuation of #276

Adds the `translator` service, which allows to easily specify strings that need to be localized. It's a good way to avoid using the `$translate` filter.

At the end of #276 we decided to hide localizations - now they are visible to the public. Not all plugins have been translated, there are still some areas available  in english only.

This PR adds translations for `comments`, `history`, `dependencyTree` directive, `MainCtrl`, `saver`, `sidepanel`, `unusedTokenHighlighter` and all navbar buttons.

What remains are: `search`, `artificialToken`, `morph`, `relation` and `sg`. There are somewhat problematic. The next PR to deal with them will explain this further.