Generalize AutocompleteSetting as OffOnSetting

[imnasnainaec]

Prequel to #3445

This enum is already used for two settings: Vernacular Autocomplete and Gloss Spelling Suggestion. It would be nice to reuse it further, namely for Merge Protection Override.

---

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.63%. Comparing base (cf6d1eb) to head (9a5d7e0).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/components/UserSettings/UserSettings.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3488      +/-   ##
==========================================
- Coverage   74.71%   74.63%   -0.09%     
==========================================
  Files         285      285              
  Lines       10996    10996              
  Branches     1337     1337              
==========================================
- Hits         8216     8207       -9     
- Misses       2398     2405       +7     
- Partials      382      384       +2     

Flag	Coverage Δ
backend	83.54% <100.00%> (-0.18%)	⬇️
frontend	66.70% <66.66%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[andracc]

Reviewed 14 of 14 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)