Fixes #478 creates QApplication for QPixmap

[kushaldas]

Without a QApplication the code will core dump as menioned
in the issue.

Description

Fixes #478

Test Plan

make test TESTS=tests/test_resources.py

Checklist

Not required.

[kushaldas]

While testing this on CI, found more issue.

$ python
Python 3.5.7 (default, Jul 13 2019, 15:29:43) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import securedrop_client.resources
>>> from PyQt5.QtGui import QIcon, QPixmap
>>> from PyQt5.QtSvg import QSvgWidget
>>> from PyQt5.QtWidgets import QApplication
>>> 
>>> 
>>> app = QApplication([])
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Reinstalling the application may fix this problem.
Aborted (core dumped)

[emkll]

Thanks @kushaldas this seems to resolve CI issues.

Built a deb locally and installed it in sd-svs, everything seems to work as expected