Merge pull request #34 from luhcas/master

Icons und Buttons auf Courseware
add CSS changes to LESS, refs #34
add CSS changes to LESS, refs #34
add CSS changes to LESS, refs #34
add global "plugin_url" helper to Mustache
move Courseware specific CSS to the block's CSS
add hrefs for navigation, refs #33
add block navigation, fixes #33
prevent SORM error with empty overview page

if json_data is not set, SORM complains that it must not be null
added video block; fixes #6
Revert "prevent SORM error with empty overview page"
The column json_data was removed in migration 006

This reverts commit f68c9db5518a55f8d8758b98de102276537fe3a7.
refactor Courseware block
refactor Container
hide edit controls for students
add JS's mustache lib
refactor VideoBlock
rename template variable
de-structure template config
add new content blocks using AJAX
fix problem with sectionless subchapters