Upgraded wagtail to 2.9.3, added image rendition caching

[gsidebo]

Pre-Flight checklist

• Testing
  • Code is tested
  • Changes have been manually tested
• Settings
  • New settings are documented and present in app.json
  • New settings have reasonable development defaults, if applicable

What are the relevant tickets?

Partially addresses #2019

What's this PR do?

Upgrades Wagtail to 2.9.3, which includes image rendition caching, and adds configuration to perform that caching

How should this be manually tested?

Since this involves a bunch of library upgrades, a good site-wide smoke test won't hurt.

To test the caching specifically:

• Start a Django shell
• Run this code snippet to check the contents of Redis:

  from django.conf import settings
  import redis
  client = redis.from_url(settings.WAGTAIL_CACHE_URL)
  client.execute_command("KEYS *")

• Load a page in the web app which has CMS images (e.g.: the catalog or a product page)
• Run the snippet above once again. There should be new keys prefixed with "wag"

Any background context you want to provide?

We are making an effort to improve caching of these images in Fastly. In an ideal world, these images wouldn't end up in the Redis cache at all. This can be considered a fallback in case the caching in Fastly falls through. If Wagtail renders an image, it will automatically be saved in the Redis cache. Subsequent requests for the image will be served from there, saving a significant amount of time/memory

[codecov-io]

Codecov Report

Merging #2029 (c456644) into master (7603fcc) will increase coverage by 9.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
+ Coverage   87.96%   97.10%   +9.14%     
==========================================
  Files         328      157     -171     
  Lines       14663     4912    -9751     
  Branches     1024        0    -1024     
==========================================
- Hits        12898     4770    -8128     
+ Misses       1529      142    -1387     
+ Partials      236        0     -236     

Impacted Files	Coverage Δ
cms/constants.py
mail/verification_api.py
courseware/apps.py
voucher/conftest.py
users/api.py
users/apps.py
fixtures/autouse.py
cms/templatetags/image_version_url.py
courses/admin.py
cms/api.py
... and 158 more

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[umarmughal824]

Is there any particular reason for adding this?

I am getting incosistent history error.

web_1     | django.db.migrations.exceptions.InconsistentMigrationHistory: Migration cms.0029_setup_course_program_index_pages is applied before its dependency wagtailcore.0043_lock_fields on database 'default'.

[gsidebo]

This was to account for a field name change in one of the Wagtail models, but I forgot to test in cases where you're not migrating from zero. I think I need to change this so it just uses some if/else logic to refer to the model field rather than adding that migration as a dependency.

[umarmughal824]

Codewise looks good but could not test because of migrations inconsistent history.

[abeglova]

LGTM