Sprint 3: GitHub releases, code cleanup.

Changes:
    * Updated Thymeleaf to 2.1.
    * Moved templates from WEB-INF to resources.
    * Implemented GitHub releases.
    * Split services into abstract interfaces and concrete components.
    * Moved beans configuration from XML into AppConfig class.
    * Merged DownloadController and MainController into AppController.
    * Split models into abstract and implementation-specific ones.
    * Temporarily removed GitHub activity service.
    * Removed wiki-based news service.
    * Moved a lot of hardcoded things into overrideable properties.
    * Added Markdown parser.
    * Removed unused libraries (incl. outdated GH client library).
    * Cleaned up templates, reduced code duplication with fragments.
    * Changed default logging settings.
    * Other code cleanup stuff.