make sure @group is a Group in avatar field - fixes #4096

otherwise it would try new_network_avatar_path
validations to prevent networks from joining networks

Also blocking committees of Networks from joining other networks.

Why? Because we need to prevent endless loops.

These are very uncommon usecases anyway and there are lot's of
ways to work around them - namely making one network a group or making
one network public and open and linking it from the other one.
upgrade to rails 2.3.15
fixed merge issue that broke the compile assets task
create secret.txt link in the right place
sort notices in proper order
prepared config/{preinitializer,boot}.rb for bundler
moved all used gems to Gemfile
setting up travis build
require rails 2.3.16 and fixed initial run order
match INSTALL with travis
specify encoding and ruby version for travis
if the database.yml is not around installing gems fails
Merge remote-tracking branch 'origin/develop' into bundler-integration
updated .travis.yml to use 'bundle' instead of 'rake gems:install'
removed 'bundle' from travis.yml again
set rails version to 2.3.16 in Gemfile
travis lacks mysql
do not forget to load the schema
changed gem install instructions so they also work with ~> versions