Added comments

Also fixes some latent, rather unimportant bugs

Also fixes some latent bugs

#125

Only define authentication backends that require external packages if
their required modules have been found.
There is no need to install packages for unused backends.

More or less directly taken from http://flask.pocoo.org/snippets/120/

close #165

close #60

Feature: Typ kann LaTeX-Template wählen

Closes #170

See merge request !8

/close #175

close #170

The row will only be shown when different templates are set up in config.py and available.
#170

Bugfix: Replaced ethernet_part with etherpad_part in line 221 to ensure that this part will only be shwon when the systems consumes the etherpad

#170

#170

Add column 'latex_template' to ProtocolType to use in the compiling process and the corresponding view

#170

#170