lawgiver update: move "Modify LaTeX Injection" permission

Move `Modify LaTeX Injection` permission to lawgiver action group `manage content settings`, according to changes in ftw.lawgiver.

@phabegger what do you think about this action group?
I tried to not introduce a new action group and reuse existing ones (and `manage` is gone in ftw.lawgiver).
`manage content settings` is maybe not perfectly right but contains similar permissions regarding superuser level :smiley: 

btw. the `Modify LaTeX Injection` permission allows to insert arbitrary LaTeX code to each object in the book which will be included in the PDF.