[Work-in-Progress] Use serializers to validate query parameters & other changes

[Deep-Chill]

• `serializers.py':

  • Added 4 query parameter serializers.

• api/billing.py:

  • Add and use query parameter serializer.

• api/metrics.py:

  • Replace GenericAPIView with APIView class
  • Add and use query parameter serializers

• api/plans.py:

  • Add and use query parameter serializers
  • Added swagger auto schema on get method

• api/roles.py:

  • Add and use query parameter serializers

  • Slightly modified POST request logic

  • mixins.py:

  • Removed period and num_periods' logics from DateRangeContextMixin since they're now applied in the PeriodSerializer

• templates/saas/profile/roles/index.html:

  • Updated variable names to match Vue component "roledescr-list"

• saas/static/js/djaodjin-saas-vue.js:

  • Minor updates

Notes:

• Could add the swagger auto schema decorator on the rest of the get methods where it isn't used as well to show query parameters. Currently it's only added in api/plans.py/PlanListCreateAPIView
• In PR #283 I changed PeriodSerializer's period field to enumfield, so there might be a merge issue.

[smirolo]

Merged #286 to replace this PR.