Add locales as a property to FlaskForm.Meta

[weinbusch]

Partly fixes #398: I propose to solve this issue by adding a locales property to FlaskForm.Meta. This property dynamically retrieves the current locale by calling i18n.get_locale. This allows DecimalField to render a localized form of the input value.

What I don't like about this solution is that the Locale object returned by get_locale needs to be converted to a language string only to be reassembled into a Locale object later by babel through DecimalField._format_decimal.

[codecov-io]

Codecov Report

Merging #399 into master will decrease coverage by 0.48%. The diff coverage is 80.76%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   98.73%   98.25%   -0.49%     
==========================================
  Files          18       18              
  Lines         949      974      +25     
  Branches       74       75       +1     
==========================================
+ Hits          937      957      +20     
- Misses         12       17       +5

Impacted Files	Coverage Δ
flask_wtf/form.py	97.46% <100%> (+0.2%)	:arrow_up:
tests/test_i18n.py	85.5% <75%> (-4.5%)	:arrow_down:
flask_wtf/csrf.py	98.1% <0%> (ø)	:arrow_up:
tests/test_csrf_extension.py	100% <0%> (ø)	:arrow_up:
tests/test_csrf_form.py	100% <0%> (ø)	:arrow_up:
tests/test_recaptcha.py	100% <0%> (ø)	:arrow_up:
tests/test_form.py	100% <0%> (ø)	:arrow_up:
tests/test_file.py	100% <0%> (ø)	:arrow_up:
... and 1 more

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