Inventory: Send email when account, about to lock, locked or unlocked

## User Story

In order to inform user of account status, send them one of three emails
@sanason manages a similar email system at Touchpoints, please consult with her!

## Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using [BDD](https://en.wikipedia.org/wiki/Behavior-driven_development#Behavioral_specifications).]

- [ ] GIVEN the system status of a user account changes from normal to about to lock, locked, or unlocked \
WHEN the status changes \
THEN send the appropriate email below

## Email 1: Account about to lock
Inventory.data.gov
User Account will lock after [x] days of non-use: [user-name] [email]
Login to Inventory.data.gov at: https://inventory.data.gov/

## Email 2: Account is locked
Inventory.data.gov
User Account is locked: [user-name] [email]
To Request unlock, please fill in this form: https://touchpoints.app.cloud.gov/touchpoints/9145dd7e

## Email 3: Account is unlocked
Inventory.data.gov
User Account is unlocked: [user-name] [email]
Login to Inventory.data.gov at: https://inventory.data.gov/

Start by locating the account-status transition logic and any existing email or notification system in the data.gov repository. Consult the referenced similar system and verify the three transitions and message contents; done means the appropriate email is sent when an account becomes about to lock, locked, or unlocked.

Written by the indexing model from the issue text.