Feature changes:

• Feature #391: add "leave page" confirmation dialog to language edit form. By @cblanken in #444
• Feature #86: add text bookmarks. By @cblanken in #439.
• Issue #449: increase source_uri book form field max length.
• Add preconfigured languages: Ainu, Bosnian, Kazakh, Macedonian, Nahuatl, Navajo, Okinawan