feat(notification): turn on digest notification on new strings by default

[nijel]

Proposed changes

This should encourage users to complete the translation they contributed to.

It applies to new users only.

Fixes #10533

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (6aee586) to head (66b37a3).
Report is 1806 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11641      +/-   ##
==========================================
- Coverage   90.82%   90.61%   -0.21%     
==========================================
  Files         554      571      +17     
  Lines       57306    58650    +1344     
  Branches     9122     9372     +250     
==========================================
+ Hits        52046    53148    +1102     
- Misses       3640     3815     +175     
- Partials     1620     1687      +67     

Files	Coverage Δ
weblate/accounts/data.py	100.00% <ø> (ø)
weblate/accounts/tests/test_notifications.py	100.00% <100.00%> (ø)
weblate/accounts/tests/test_views.py	100.00% <100.00%> (ø)
weblate/api/tests.py	100.00% <100.00%> (ø)

... and 247 files with indirect coverage changes