Add styles and CZ.NIC branding

[jiri-janousek]

Closes #59

CZ.NIC styles and logo are in eidas_node.cznic submodule and must be enabled in settings:

• Add from eidas_node import cznic.
• Add STATICFILES_DIRS = [cznic.STATIC_DIR].
• Add TEMPLATES['DIRS'] = [cznic.TEMPLATE_DIR].

Instead, we could style generic templates and add configuration options for:

• logo URL (but where should the file be stored?).
• info text ("Service provided by CZ.NIC, CZ domain registry.").

@jtalir thinks that the users of django-eidas-specific-node are likely to use their own template anyway and we can change if the need arises.

[codecov-io]

Codecov Report

Merging #61 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #61   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          41     42    +1     
  Lines        3252   3256    +4     
  Branches      195    195           
=====================================
+ Hits         3252   3256    +4

Impacted Files	Coverage Δ
eidas_node/proxy_service/views.py	100% <100%> (ø)	:arrow_up:
eidas_node/cznic/__init__.py	100% <100%> (ø)
eidas_node/tests/proxy_service/test_views.py	100% <100%> (ø)	:arrow_up:

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