deposit: Explicit deposit type, either 'meta' or 'code' deposit

This should ease deposit listing in whatever forms (backend db read or client consuming
deposit listing).

Deposit types stand for:

• meta: metadata-only deposit
• code: content deposit

This commit includes a migration schema script which adds a new column 'type'. The
script is also in charge of migration existing data with the right type values..

Related to T3677