DM: Session lists - use infinite/'Load more' scrolling instead of pagination

[kerryarchibald]

For https://github.com/matrix-org/matrix-authentication-service/issues/1492

AC:

• compatibility, oauth2 and browser session lists show a 'load more' button at the bottom when there are more sessions
• clicking the button loads another page and appends it to the list

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: https://github.com/element-hq/matrix-authentication-service/issues/1532