bugfix: populate source list immediately upon login (#577)

[redshiftzero]

Description

Closes #577

I investigated what was going on with #577, turns out it's a fast fix:
the controller's update_sources() method is used to display in the
source list whatever is contained in local storage. If we wait until
after sync_api is called before calling it (what we're doing on master),
there will be a delay of potentially several seconds until the round-trip
to the SecureDrop server completes. In this commit we just call the
update_sources() method immediately upon login.

Test Plan

0. Reproduce source list is populating very slowly in subsequent runs of the application #577, i.e. confirm the the source list populates slowly (will only happen if you're going over tor)
1. Check out this branch and login
2. Confirm the source list appears populated immediately upon login

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

[kushaldas]

Testing this one (after the Qubes update is done).

[kushaldas]

• Confirm the source list appears populated immediately upon login

This works as defined in Qubes. Approved.