Centralize resource creation logic & add basic input validation.

Issues this PR fixes:

• inconsistent logging between graph & rest
• parity between inputs for graph & rest resource creation
  • ex. graph let you specify group enabled/disabled, whereas rest always set as enabled
• can no longer specify unintended fields in rest such as createdAt or fingerprint
• receiver creation through graph lacked schema validation
• string/id fields cannot be blank (including whitespace, inputs trimmed)
• groups need at least one receiver id

E2e tests are closer to passing, now the only failures should be:

• TestCreateInvalidGroup/non-existent_receiver - creating group with invalid receiver yields 5xx response instead of 4xx. This happens because there's no code checks, only a failed foreign key constraint from the db layer
• TestToggleGroup - UpdatedAt field never updated. Gorm should do this automatically but is "broken" by us using types besides time.Time (we use a custom graph scalar)

Closes #70