Make lessons without subjects available

[techmaved]

Fixes: #129

Motivation:

Some lessons in my time table don't have a valid subject. The underlying webuntis library throws an error when accessing the subjects of a lesson. I wanted to introduce a setting that controls if these lessons should be ignored or not.

Changes

• add new filter setting to 'use invalid subjects'
• check setting in check_lesson method when error is thrown
• add calendar event for invalid lessons

[JonasJoKuJonas]

Ich werde, wenn ich wieder Zuhause bin den code testen und dann ein Update pushen.

[JonasJoKuJonas]

implementet via merge of local branche