allow extra env, extra envFrom, extra InitContainers, and extra sidecars

[jessebot]

Adds the following for more flexibility of this helm chart:

# -- template out extra environment variables from ConfigMaps or Secrets
extraEnv: []

# -- template out extra enviornment variables
extraEnvFrom: []

# -- set extra init containers
extraInitContainers: []

# -- set sidecar containers to run along side the pixelfed container
extraContainers: []

This should make it easier to test other images that use a .env file :)