Show full timestamp in UI

[philmcmahon]

Description

Fixes freedomofpress#1563

Building on freedomofpress#1626 this change modifies securedrop-client so that it shows the full timestamp rather than just the date of the last source activity.

Once concern noted elsewhere about this date is that it is not the last updated (as the user would expect) - it's the last source activity time. I've added a tooltip to this effect but I don't think that's sufficient. However, we're already showing the date, adding the time doesn't make it any more 'wrong' so this feels like a helpful improvement.

I've got two other date/time related PRs stacked up:

• Localise dates shown in UI using QTimeZone freedomofpress/securedrop-client#1626 (has been partly reviewed)
• When formatting dates, use system locale #8 (currently against our fork whilst waiting on the above)

With that in mind, I think we could merge this to our fork potentially and bundle with our next release, whilst separately proposing changes to the main freedomofpress repo once those other changes have been discussed/approved.

Test Plan

I've tested this on qubes and macos
Qubes:

Macos:

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