schema: switch from Option<NonBlankString> to String for Signs

[Khoyo]

No description provided.

[flomonster]

LGTM

[codecov]

Codecov Report

Merging #5279 (e2375e0) into dev (eb5d93f) will increase coverage by 0.01%.
Report is 2 commits behind head on dev.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                dev    #5279      +/-   ##
============================================
+ Coverage     18.66%   18.68%   +0.01%     
  Complexity     2320     2320              
============================================
  Files           856      856              
  Lines        104009   103926      -83     
  Branches       2410     2410              
============================================
- Hits          19416    19414       -2     
+ Misses        83253    83172      -81     
  Partials       1340     1340              

Flag	Coverage Δ
core	78.98% <ø> (ø)
editoast	69.77% <ø> (-0.03%)	⬇️
front	7.05% <ø> (+<0.01%)	⬆️

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Files	Coverage Δ
editoast/src/schema/mod.rs	94.20% <ø> (ø)

... and 6 files with indirect coverage changes