Simple account management

[mfornos]

• [x] #103
• [x] Minimal account model
• [x] CRUD account (API and persistence)
• [x] Invite accounts (i.e. token + account)
• [x] Issue token for account
• [x] Revoke token for account
• [x] #105
• [x] #106
• [x] Testing
• [x] #108

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NOTES

Just the minimal backend. The UI is for next milestones.