Fixed translation script.
Replaced <message-dialog> with <loot-message-dialog>.

The latter extends <loot-dialog>. Also moved common <paper-dialog>
attributes into <loot-dialog>'s default set.
Replaced plugin-li element with <loot-plugin-item>.

The latter extends <paper-item>. Also split Polymer CSS overrides into a
new CSS file, and fixed sidebar plugin item display.