Update Babel from 2.5.1 to 2.9.1

[zenmonkeykstop]

Status

Ready for review / Work in progress

Description of Changes

Fixes #5962

Updates Babel from 2.5.1 to 2.9.1

Testing

• CI is passing
• dependencies other than babel are unaffected.

Deployment

Deployed with securedrop-app-code install/update

Checklist

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[rmol]

LGTM. Things still work, and the test changes here are necessary adjustments for new Babel behavior -- adding group separators, skipping empty messages when writing .mo files.

I tested the new fallback behavior when a message is simply missing from the .mo file, instead of being present with an empty message string, and nothing broke. You just get English, as expected.