Change IRC references to Slack

AC-518 updates `button-name` todo; needs TinyMCE work

Bump UI Toolkit version to 1.4.1

Required for the breadcrumbs work I will be doing later this week. Also should issues we are seeing with NPM this morning.

Program detail page touchups

[PERF-344] Add versioning of cached course assets to allow graceful cache invalidation

When releasing the versioned assets work, we stumbled on a problem with old pickled
versions of the StaticContent objects residing in cache, which triggered a bug in the
code. Not wanting to blow away all cached items, we ended up having to revert and add
in some backwards-compatible helper code to ease the transition.

With this, we'll now utilize the version argument that Django's caching interface
allows, in conjunction with a constant value that can be modified when breaking changes
are being made, to let us gracefully ignore older cached course assets.

Convert Dates for Time Zone

Converts the dates on the dashboard, sidebar navigation, and important course dates to user specified time zone.

Center cards and add padding around logos. ECOM-4871.

Fix issues with topic truncation.

Release

Replacing relative imports by absolute imports for all files into com…

Fix footer test failures

Add upgrade section to program detail course cards.

Displays a message and link when a user is enrolled in a program's course run but must upgrade in order to be eligible for the program certificate. ECOM-4220.

Fix `bulk_change_enrollment` command to emit event

ECOM-4775

This change improves grading performance by combining what had previously been
multiple course tree traversals into a single traversal with combined
filtering logic.

Make this message the same as another almost identical message

Update translations (autogenerated message)

removes text value from span that is intended only to receive focus

AC-519 removing list from ignored a11y check

Show message on progress page when user is in audit mode.

Fix diff cover args

Hamzamunir/tnl 4141 problem creation empty answer not allowed

WL-416: Multi-site comprehensive theming

2. Update COMPREHNSIVE_THEME_DIR to COMPREHENSIVE_THEME_DIRS
3. Update paver commands to support multi theme dirs
4. Updating template loaders
5. Add ENABLE_COMPREHENSIVE_THEMING flag to enable or disable theming via settings
6. Update tests
7. Add backward compatibility for COMPREHEHNSIVE_THEME_DIR

Merge release into master