Integrations support + Prejournal, Tiki

[gsvarovsky]

Closes #63
Closes #62

TODO:

• Prejournal Movement IDs Include movementId in JSON response pondersource/prejournal#127
  (Also changed worked-hours structure, worked-hours command not usable when user name != worker name pondersource/prejournal#143)
• Prejournal external IDs Ability to specify external identifier in worked-hours command pondersource/prejournal#129) Include external IDs in Prejournal integration #69
• Project integrations (not just timesheets) Integrations don't function when added to Projects #70
• Integration per-instance config (e.g. prejournal/tiki host, tiki tracker) Per-instance integration configuration #71
• Node leader election per integration Gateway node leader for integrations #72
• Export timesheets to agreed external systems #64 (iCal) actually, import is wanted, and should be a separate PR Import timesheets from agreed external systems #24
• Integration garbage collection adding to tech debt