Prevent creation of two templates with same title.

[dracos]

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Fixes https:/mysociety/fixmystreet-commercial/issues/1364

[codecov]

Codecov Report

Merging #2471 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2471      +/-   ##
==========================================
+ Coverage   81.29%   81.31%   +0.01%     
==========================================
  Files         197      197              
  Lines       12639    12645       +6     
  Branches     2348     2350       +2     
==========================================
+ Hits        10275    10282       +7     
+ Misses       1579     1578       -1     
  Partials      785      785

Impacted Files	Coverage Δ
perllib/FixMyStreet/App/Controller/Admin.pm	80.48% <100%> (+0.38%)	⬆️

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[davea]

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