Add fixture for collections

[rgaiacs]

@costascaio have a look at src/fixtures/ and src/fixtures2seeders/. You need to create

• src/fixtures/collections.json
• src/fixtures2seeders/20200801232000-collections.js

Please note that all files in fixtures/ are generated by src/fixtures/db2fixtures.sh and because of it you must edit src/fixtures/db2fixtures.sh.

[rgaiacs]

src/fixtures/db2fixtures.sh requires CRICDatabase/searchable-image-database#8.

[rgaiacs]

@costascaio you must implement this as part of #141.