Cannot delete branch with slash in name after PR

[Aarup]

• Gitea version (or commit ref):1.1.0
• Git version:
• Operating system: Ubuntu
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL) https://try.gitea.io/username/branch-slash/pulls/1
  • No
  • Not relevant
• Log gist:
  https://gist.github.com/Aarup/75908798b147e91ffb1133357acb44a1

Description

If a branch has has a slash in the name like

• bug/bugsId
• feature/featureName
  It is impossible to delete this branch using the web UI after a PR has been merged into the repository.
  When clicking the "Delete branch" button a generic popup comes, but after entering username and password nothing happens - a 404 is received from the post

Screenshots

[lunny]

#1689, it seems Gitea has a different URL with github's URL on this feature.