Campaign(s) remaining (redis)

[elpiel]

Since we need to use the calls to redis in multiple occasions ( #413 & #408 )

Create a single struct (sentry::db::campaign::CampaignRemaining) that handles the key & have all required calls we need for Redis (for creating & modifying campaign as well as Events recording).
This will remove duplicate logic and some error prone places (like having the redis key twice on 2 different locations)

[elpiel]

CampaignRemaining is now used in the following places:

• Campaign insert events route uses the Issue 382 campaign insert events #413
• Create/Modify campaign routes V5: Authenticated POST route to modify/submit campaigns #408 (changes made in Issue #415 Create/Modify Campaigns - Redis and tests #418 )