Verify Project restrictions when handling incoming payloads; not just at validation

Signed-off-by: David Greaves <david.greaves@jolla.com>
Add some explanatory text for Projects

Signed-off-by: David Greaves <david.greaves@jolla.com>
Add migration for uniqueness constraint
Update process to use log_channel and default to #mer-boss (as per BUILD process)

Signed-off-by: David Greaves <david.greaves@jolla.com>
Revert create_project to use non-DRY parse_bz_config until python-boss-common is updated in public

Signed-off-by: David Greaves <david.greaves@jolla.com>
Add missing VCSNameSpace import and a comment for placeholder mappings

Signed-off-by: David Greaves <david.greaves@jolla.com>