Fix poll preview and localize options

[frcroth]

This PR:

• fixes poll previews
• Allows localization of options
  • These are now actually shown
• Changes poll behavior so that all poll localizations use a canonical poll in the default locale for can_vote and cast_vote
• Preview should see options and not be able to vote.
• Poll properties are kept in sync

For testing:
Create a poll before applying migrations to test the migration.

Fixes #542
Fixes #527

[coveralls]

Pull Request Test Coverage Report for Build 18062140806

Details

• 180 of 202 (89.11%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+1.2%) to 74.547%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
myhpi/polls/templatetags/polls.py	17	21	80.95%
myhpi/polls/migrations/0013_assign_translation_keys.py	12	18	66.67%
myhpi/polls/models.py	46	58	79.31%

Files with Coverage Reduction	New Missed Lines	%
myhpi/polls/models.py	1	87.83%

Totals
Change from base Build 16481471613:	1.2%
Covered Lines:	1728
Relevant Lines:	2318

---

💛 - Coveralls

[frcroth]

Oh, I just noticed a regression in your PR: Ordering choices/options does not on both poll types work after applying your migrations.

I suspect this is related to clearing the model options in migration 0012, at least the docs specify that these options are used for ordering (among others) and migration 0011 sets "ordering": ["sort_order"]

Good catch!
If I understand it correctly, the Meta class of the model was now taken from TranslatableMixin and no longer from Orderable. So I added it explicitly with the two Metas merged.

[frcroth]

Lgtm, thank you for addressing all the poll issues!

Awesome, thank you for your thorough testing and feedback!