Allow whitelist admins to add new tranches

[dusan-maksimovic]

Closes #52

• Changed tranches initialization- now we pass only names and IDs are assigned automatically
• Duplicate tranche names are now forbidden
• Added new execute message that allows whitelist admins to add new tranches

[p-offtermatt]

Nice work.
Some thoughts:

• I think we can probably do without a DeleteTranche function
• Could we add the metadata field back to the tranches? I think it's probably important eventually, and I want to avoid a need to do hacky things like putting all data in the name field. Might also be useful to have a message to edit a tranche name and description for a given id.

[p-offtermatt]

LGTM, I think the validation is good. Just a nit about the response, but otherwise I think we can merge