change answer correction indicators, fixes #69
Merge pull request #80 from luhcas/master

Optische Anpassungen an der Navigation
keine scrollbars im videoblock; fixes #77
don't change other video-blocks; fixes #76
some styling changes to the video block
make blocks deleteable
add icon for block deletion
refactor TestBlock templates, add answer strategies to ease the implementation of additional exercise types, refs #78
move css to less-files
add yes/no exercises, refs #78
Some improvements to answers strategies (refs #78):

* renamed AbstractAnswersStrategy to AnswersStrategy
* base answers strategy class is no longer abstract
* add default implementation in AnswersStrategy for all methods of AnswersStrategyInterface
* add a factory method to AnswersStrategy to receive concrete answers strategies for a given Vips exercise
add a method to retrieve the solution of an exercise for a test and a user in the Vips internal format, fix a bug in the handling of solutions, refs #78
show edit area for sections, refs #42
fix button
extend the strategy interface to optionally handle the answers given by a user, refs #78
add strategy for free text exercises, refs #78
Darstellungsfehler in Navigation behoben