Introduce a new CredentialTemplate model

Credentials now have a required CredentialType, which defines inputs
(i.e., username, password) and injectors (i.e., assign the username to
SOME_ENV_VARIABLE at job runtime)

This commit only implements the model changes necessary to support the
new inputs model, and includes code for the credential serializer that
allows backwards-compatible support for /api/v1/credentials/; tasks.py
still needs to be updated to actually respect CredentialType injectors.

This change *will* break the UI for credentials (because it needs to be
updated to use the new v2 endpoint).

see: #5877
see: #5876
see: #5805

tools/data_generators/rbac_dummy_data_generator.py

@@ -321,7 +321,7 @@ try:
                         name='%s Credential %d User %d' % (prefix, credential_id, user_idx),
                         defaults=dict(created_by=next(creator_gen),
                                       modified_by=next(modifier_gen)),
-                        kind='ssh'
+                        credential_type=CredentialType.from_v1_kind('ssh')
                     )
                     credential.admin_role.members.add(user)
                     credentials.append(credential)
@@ -344,7 +344,7 @@ try:
                         name='%s Credential %d team %d' % (prefix, credential_id, team_idx),
                         defaults=dict(created_by=next(creator_gen),
                                       modified_by=next(modifier_gen)),
-                        kind='ssh'
+                        credential_type=CredentialType.from_v1_kind('ssh')
                     )
                     credential.admin_role.parents.add(team.member_role)
                     credentials.append(credential)