Remove extra slash from twitter card

[techknowlogick]

on try.gitea.io the current looks like: <meta property="og:image" content="https://try.gitea.io//img/gitea-lg.png" />

This PR removes the extra slash from the AppUrl

cc: @ashimokawa

[codecov-io]

Codecov Report

Merging #6619 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6619      +/-   ##
==========================================
+ Coverage   40.46%   40.47%   +0.01%     
==========================================
  Files         406      406              
  Lines       54484    54484              
==========================================
+ Hits        22045    22052       +7     
+ Misses      29402    29395       -7     
  Partials     3037     3037

Impacted Files	Coverage Δ
routers/repo/view.go	42.07% <0%> (+0.99%)	⬆️
modules/log/event.go	65.98% <0%> (+1.52%)	⬆️

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