Channel improvements

Changes:

• Add header with channel name to manage channel and edit channel subscription pages
• Rename routes:
  • /main/channels/<channel>/manage to channel administration (by owners/admins) (instead of /main/channels/<channel>)
  • /main/channels/<channel> to manage channel subscription (instead of /main/channels/<channel>/notifications)
    • also adds /main/channels/<channel>#notifications and /main/channels/<channel>#preferences