Add wagtail-localize-smartling to the project, for Smartling L10N support

[stevejalim]

One-line summary

This changeset drops in wagtail-localize-smartling, which will flow strings from Wagtail to Smartling for L10N and then back again.

Significant changes and points to review

0.2.3 is an early-ish, Minimum Lovable Product version, and there may well be some fast-follow changes from the vendor - those are in discussion at the mo, and we can pick up anything else we want, as it's "our" codebase now.

I've tested it manually, locally, for now, with sandbox-y API keys and it works for the core cases. There are some rough edges but we'll sort them out.

The infra PR needed to accompany these changes will need to:

1. Set the appropriate env vars as documented
2. Set a cron job that runs ./manage.py sync_smartling every X minutes (15?). Doing it as a separate job on a cron means we don't risk timeouts in the web UI.

Issue / Bugzilla link

Resolves #14774

Testing

N/A for now - I'll record a video demo, but waiting to see if there'll be a 0.2.4 soon (or not yet)

[stevejalim]

Review tagging is more about awareness than close reviewing right now. Good to get this merged (when the infra is ready)

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.38%. Comparing base (fe9144d) to head (ed6edbb).
Report is 5 commits behind head on main.

Files	Patch %	Lines
bedrock/settings/base.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14794      +/-   ##
==========================================
+ Coverage   77.29%   77.38%   +0.09%     
==========================================
  Files         160      161       +1     
  Lines        8281     8306      +25     
==========================================
+ Hits         6401     6428      +27     
+ Misses       1880     1878       -2     

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[stevejalim]

Secrets deployed to dev and will not crash when code reaches Stage or Prod because we have non-exploding (but deliberately invalid) defaults set of setme

[pmac]

r+wc

everything other than the question about env vars looks good.

[robhudson]

Just noticing the above text looks like it's wrapped whereas this new text isn't.