Availabilities CRUD

[v-pak]

Still need to fix Auth with handlers, handing this off to @Plebbaroni as per conversation in dev session

• 2 Postgres tables for storing availabilities
  • user_campaign_availabilities - for user_id and campaign_id
  • availability_slots - for the actual timeslots
  • Done in this way to avoid changing user_id and/or campaign_id for every timeslot when only one change is needed
• Models that modify these tables
• 2 Handlers calling the models
  • get - gets all the timeslots for a given user_id and campaign_id
  • update - given a set of availabilities, updates the database to match for a given user_id and campaign_id
• "/api/v1/availabilities/:user_id/:campaign_id" route, implementing a get and a patch router for the corresponding handlers

[Plebbaroni]

Good work! Not gonna merge just yet as I wanna do the handlers and wanna keep this as a WIP. TYSM for the work on this PR though! :D

[v-pak]

Yep! Handlers are unfinished and need proper auth code rather than the current placeholders, so this should stay as a draft until that's complete.

Thank you for the review!