Show full timestamp for message last updated time

[philmcmahon]

Description

This change modifies the securedrop UI so that the time of the most recent message sent by the source is shown in the UI (as opposed to just the month an the day).

There's quite a bit of prior discussion on whether this is a good idea or not - see the issues below for furth details. From the guardian's perspective, we think having the time is extremely valuable when working in a multi user environment

Replaces #11 and is related to:

• Show last_updated time as well as date in sources list freedomofpress/securedrop-client#1563
• Implement "last activity" timestamp freedomofpress/securedrop-client#538

It looks like this:

Test Plan

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration is self-contained and applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed