Allow setting custom names on sessions

[sandhose]

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This does a few things:

• on the compat /login API, it gathers the initial_device_display_name and sets it as the session human_name
• that human_name is transmitted to the homeserver when creating the device
• similarly, we now compute a initial device name for OAuth 2.0 sessions, which usually looks like Element on Safari for macOS
• to be able to do that, we now track the locale the browser had when we started the authorisation code grant
• it also introduces the custom human_name on oauth2 sessions
• those human_name are shown on the frontend (& GraphQL & admin API)
• and there is an edit button on the frontend to set a custom name for the session

Notable omissions:

• we don't compute a session name for device code grants. We'd also need to track the locale to do that
• we don't update the device display name on device list sync. They can very much get out of sync, especially if the device name gets updated in the client, which is likely fine

Fixes #2366
Related to #3919

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