Merge branch 'issue_3780' into 'master'

Downcased user or email search for avatar_icon.

GitLab users are defined with their mail address which is enforced to be lower case. When a commit is listed in the history whose committer mail address is not written in all lower case, the corresponding GitLab user won't be found because the search is case sensitive resp. the mail address to search for not downcased.

Closes #3780 

See merge request !2234

app/helpers/application_helper.rb

@@ -72,7 +72,7 @@ module ApplicationHelper
     if user_or_email.is_a?(User)
       user = user_or_email
     else
-      user = User.find_by(email: user_or_email)
+      user = User.find_by(email: user_or_email.downcase)
     end
 
     if user