Customization

This push request lays the ground for the issue #311 

* rename logo images to avoid hardcoding company name in the file's name
* move alt attribute for logo img tag to message bundle
* allow adding new pages with url like `tatami/mypage`
* allow restricting available themes (and adding new themes)

The actual customization is demonstrated in https://github.com/PierreRust/tatamiCustomization and is a simple use of maven war overlay to override some tatami files.