User session/device list does not show all sessions

### Description

When I view the details of my own user (in the sidebar), e.g. by clicking on my own username in a room userlist, or at https://riot.im/app/#/user/@matthijs:matrix.org (which is also where the "Verify where you're logged in" popup/toast sends you), I see a list of sessions:


(Here, I already clicked "2 verified sessions" to show the complete session list).

However, it seems this list is incomplete. When looking at the account settings, "Security & Privacy" section, there is a much longer list:


(this just shows the top half of the list, there's more session below).

I can imagine that the sidebar only shows a limited list of sessions, maybe only recent ones? This is probably sane, but it would be good if there was some indication of this, e.g. by adding "8 more sessions not shown" or so.

### Version information

- **Platform**: Web
- **Browser**: Firefox 78
- **OS**: Ubuntu 19.10.
- **URL**: riot.im/app

Start by reproducing the session lists in the user sidebar and in Account Settings > Security & Privacy, then compare how each view obtains and limits sessions. Done means the sidebar shows all sessions or clearly indicates how many are omitted, with the two views no longer appearing inconsistent.

Written by the indexing model from the issue text.