Add databases and tools fields to TrainingEvent model

[joncison]

databases
Databases to which the training event is relevant.
biotoolsid
0 to many

tools
Tools to which the training event is relevant.
biotoolsid
0 to many

Must first settle how to model databases and tools.

[joncison]

This depends on #39

[bryan-brancotte]

database will be tacked if needed with #228 #239

tool are not considered yet