In order to remove the deprecated flag_undefined_default=True
argument, this commit updates the following flags to always be
enabled using a new temporary class:

- course_experience.course_outline_page
- course_experience.unified_course_tab

Adds a temporary setting `USE_DEFAULT_TRUE_NAMESPACE`,
to enable a monitored rollout of this change.

TNL-7061 is the ticket where these flags will actually be
removed. This requires more careful work including removing
all dead code, and potentially refactoring tests that were
testing shared functionality, but only when the flag was
False.

ARCHBOM-1316

- Looks at masquerading config for dates, outline, metadata, and
  celebration APIs in course_home_api / courseware_api.
- Consolidates and cleans up places we check whether masquerading
  gives us full access to a course.

[BD-10] [DEPR-83]

- add serializer mixin to expose date banner info
- add endpoint to reset deadlines for a sepcific course

Simplify how the course-outline template context is constructed to bring it into parity with other template contexts

Only actually reset user schedule if they have missed deadlines and not missed any gated content deadlines

Reducing dates widget to one assignment. Adding link to dates
page in the dates widget

Refactor dates_banner_should_display to only return a single value, and to have a clearer set of conditions

This reverts commit c3ec8b83, reversing
changes made to 4252b149.

Only prompt to upgrade in order to completed graded content (or shift dates) when content_type_gating is enabled

This takes over some of the logic in the is_enabled property which
previously checked two things: (1) is the date time-sensitive and
(2) is the date even applicable to this course.

Now, is_enabled is only responsible for time-sensitive checks and
the new is_allowed property checks applicability.

In this way, we can more cleanly separate whether a date block
should show up on the dates sidebar (is_enabled and is_allowed)
and the dates tab (is_allowed).

Use a fixed time for course duration limits, so that schedule extensions don't extend FBE duration limits

- use new dates banner template on course outline page
- remove old banner from main.html
- let dates tab use new dates banner template
- remove dates banner completely from the courseware problem view
  on the web app
- use new banner on the courseware problem view on the mobile app
- update banner util to use get_course_blocks

- add banner to dates page when deadlines are missed

Add financial assistance link in course tools where available, so this will be more visible to learners

This reverts commit cbb4cfb9.

Removed use of the 'shorten_welcome_message' waffle flag

AA-108

Add financial assistance link in course tools for eligible courses, so it'll be more visible to learners

[AA-138]

In the course outline, if a subsection has any graded content,
show a graded icon (the pencil icon).

Also, show the graded icon for LTI xmodule units (xblocks is a
different repo, but will get same treatment).

AA-75