When joining an upgrade room, its notification level should be the same as the original

[eras]

Description

When a channel (Synapse Admins) was updated from v1->v5, the newly joined room does not preserve the previous channel's notification settings.

Steps to reproduce

• Set the room's notification setting to "Mentions only"
• Upgrade the room from v1 to v5 (I guess someone else may need to do this)
• Click the button in the bottom allowing to view the updated room
• Join the room
• Check the room notification settings from the room list
• Notice that the notification setting is "All messages"

I would expect the notification level to be the same as of the original room, "Mentions only".

Version information

• Platform: web
• Browser: Firefox 66.0.3 (64-bit)
• OS: Ubuntu
• URL: riot.im/staging
• OS: Windows, macOS, Ubuntu, Arch Linux, etc?
• Version: 1.2.2-rc.1

[turt2live]

This is a synapse issue: matrix-org/synapse#5295