fix(discovery): disable edit_template if format does not support it

[nijel]

Proposed changes

Otherwise user gets an error which is hard to understand.

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

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.14%. Comparing base (53c5ad5) to head (2472411).
Report is 3314 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/trans/discovery.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12855      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         596      596              
  Lines       61191    61214      +23     
  Branches     6349     6350       +1     
==========================================
+ Hits        55780    55792      +12     
- Misses       3758     3767       +9     
- Partials     1653     1655       +2     

Files with missing lines	Coverage Δ
weblate/trans/discovery.py	91.05% <71.42%> (-0.76%)	⬇️

... and 2 files with indirect coverage changes