Improvements in the DatabaseCleaner

[Ana06]

Move all what is related to the DatabaseCleaner to the database_cleaner.rb file. Load the seeds only when they have been deleted and not before every test.

[ChrisBr]

LGTM

[hennevogel]

truncation != transaction

[Ana06]

@hennevogel I know it, but it happens something strange, after truncating the database and loading the seeds and before the first test is executed some data is introduced in the database. I have no idea of where this data comes from.

So this strange data keeps there until a truncation is executed (after a feature). If a test query the information in the tables where strange data is introduced (for example User and Conference) before they are truncated it fails.

I added some puts here that illustrate what happens:

https://travis-ci.org/Ana06/osem/builds/191338450

This only happens in Travis, locally it works.

So the second commit (to truncate the database before each test) is just a quick solution until we find out why is this happening. Because otherwise merging the opened PR seems no to be much safe.