[WIP] Address issues of PR 216

[shun-liang]

This PR is based on #216, and is to address the comments raised in that PR.

The diff between this PR and #216 can be seen at Andrey-Skvortsov#1

Done:

• Undo version bumping
• Use the latest PyPI release of https://github.com/onysos/django-composite-foreignkey, which now supports Django 2.0: Add Django 2 support, bump to v1.0.1 onysos/django-composite-foreignkey#13 (comment)
• Fix "fetching too much translation paths" problem for get_extra_related_translation_paths
• Add tests for SelectRelatedTranslationsQuerySetMixin

TODO:

• Fix other problems raised in the review of Reducing number of db queries by using select_related data #216
• (Maybe) make things easier to review

[codecov]

Codecov Report

Merging #219 into master will increase coverage by 1.02%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   86.19%   87.22%   +1.02%     
==========================================
  Files          30       31       +1     
  Lines        1797     1933     +136     
==========================================
+ Hits         1549     1686     +137     
+ Misses        248      247       -1

Impacted Files	Coverage Δ
example/article/tests.py	98.8% <ø> (ø)	⬆️
parler/tests/test_model_construction.py	93.42% <100%> (+1.01%)	⬆️
parler/tests/test_querysets.py	100% <100%> (ø)
parler/tests/test_model_inheritance.py	100% <100%> (ø)	⬆️
parler/tests/test_query_count.py	96.49% <89.47%> (-3.51%)	⬇️
parler/tests/testapp/models.py	94.32% <95.23%> (+0.15%)	⬆️
parler/utils/fields.py	96.29% <96.29%> (ø)
parler/managers.py	91.21% <98.52%> (+6.21%)	⬆️
parler/cache.py	65.62% <0%> (-3.13%)	⬇️
... and 4 more

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