Add the ability to use multiple labels as filters

[Kuermel]

• Git version: 1.3.2
• Operating system: Linux
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Description

In Github there is the possibility to select several labels as filters at the same time. This feature is useful if an issue has multiple labels.

Screenshots

[Kuermel]

#3407 Add "no label" and "no milestone" filters goes in a similar direction