no longer delete all source widgets each update

[sssoleileraaa]

Description

Fixes #905
Fixes #352

We currently delete all source widgets and recreated them every time a user logs in, a user deletes a source, and whenever the client syncs with the server.

This PR makes it so only add source widgets when there are new sources and delete source widgets when the source has been deleted.

Performance testing on a large source list will need to be done over the next day or so. Also we should check if this helps with the flicker issue.

Test Plan

Make sure there are no regressions. This is a refactor so the source list should behave the same.

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

[sssoleileraaa]

rebased to keep this still ready for review

[sssoleileraaa]

i gathered some perf data and it looks like update_sources which updates the source list is now dramatically faster, and you can see how the GUI doesn't freeze up anymore when you select between sources with a source list of 400.

n=100 sources

before

0.46784329414367676
0.2571258544921875
0.2532978057861328
0.2886519432067871
0.2683417797088623
0.2562129497528076
0.2976980209350586
0.2644331455230713
0.2614705562591553
0.261214017868042
0.26711487770080566
0.2992699146270752
0.2701082229614258

after

0.4843146800994873
0.0020008087158203125
0.0019850730895996094
0.002037525177001953
0.002022981643676758
0.0020155906677246094
0.0021219253540039062
0.0020911693572998047
0.002016782760620117
0.0019176006317138672
0.002031087875366211
0.0019462108612060547
0.002020120620727539

n=400 sources

before

0.5068690776824951
1.8598730564117432
3.211728811264038
3.6987013816833496
3.1780104637145996
4.074876308441162
3.9540183544158936
(i had to stop because each sync was taking so long)

after

1.827559471130371
0.008049488067626953
0.01360464096069336
0.01093912124633789
0.01216745376586914
0.00998687744140625
0.010653972625732422
0.011968851089477539
0.010234355926513672
0.011265039443969727
0.016140460968017578
0.011561155319213867
0.010588884353637695
0.012189388275146484
0.011542081832885742
0.011640310287475586
0.012189626693725586
0.010349273681640625

[redshiftzero]

this looks great, a few thoughts inline

[redshiftzero]

confirming: since we no longer need to redraw the whole view, we don't need this logic to preserve the current source selection

[redshiftzero]

ensuring that new sources are placed at the top of the list ✅

[sssoleileraaa]

just need to update tests and rebase but i addressed @redshiftzero's PR comment https://github.com/freedomofpress/securedrop-client/pull/893/files#r390031326 about this introducing an ObjectionDeletionError by storing the source uuid on the widget so that we can delete the widget without access the db source object. as follow-up we can completely remove the source object from the source widget, but this will require updating set_snippet and a few other areas of the code.

[sssoleileraaa]

okay this is ready now

[redshiftzero]

my comments are addressed, thanks