Issue 5 health function

[elpiel]

Part of #5

• [x] is_finalized()
• [x] get_status (needs testing)
• [x] is_offline (needs testing)
• [x] Cache::finalize_campaign
• [x] bugfix Cache::initialize
• [x] SentryApi::get_last_approved
• [x] httptest setup

[elpiel]

I am merging this one as it became too big, I will add tasks to #5 to account for the missing tests, etc.