Add occurred_at column

[michplunkett]

Fixes issue

#980

Description of Changes

When both the time and date fields are filled in, we migrate them to the occurred_at column which is stored in UTC.

Screenshots

Before migration:

After initial upgrade:

After downgrade:

Final upgrade:

Incident listing:

Tests and linting

• This branch is up-to-date with the develop branch.
• pytest passes on my local development environment.
• pre-commit passes on my local development environment.

DB Migration Output

$ flask db upgrade
[2023-07-27 20:20:06,782] INFO in __init__: OpenOversight startup
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 18f43ac4622f -> 9fa948bcea25, change incident date and time to timestamptz
$ flask db downgrade
[2023-07-27 20:36:20,895] INFO in __init__: OpenOversight startup
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running downgrade 9fa948bcea25 -> 18f43ac4622f, change incident date and time to timestamptz
$ flask db upgrade
[2023-07-27 20:36:49,910] INFO in __init__: OpenOversight startup
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 18f43ac4622f -> 9fa948bcea25, change incident date and time to timestamptz
$ 

[michplunkett]

I think I had a much simpler understanding of this and was thinking of this as if we were handing the backend a UTC timestamp. I am modifying this PR a good amount. I'll reach out when it's good.

[michplunkett]

I'm gonna knock this one out on Monday when my mind is a little clearer.

[michplunkett]

I'm gonna restart this feature in the following PR: #1024