Add e2e tests for lazy loading scenarios

[bwindels]

to register many users without having to spin up a browser for each, use shared secret registration which lets you register users directly via API if you know the secret in the synapse config.

https://github.com/matrix-org/synapse/blob/master/docs/admin_api/register_api.rst

Test scenarios:

• Bob creates room, Charly{1..10} join, Bob types 20 messages, Alice joins and checks she sees all charlys with correct display name in the memberlist & timeline.
• Bob joins room. Alice joins room & waits for members to load and then goes offline. Charly joins. Now Bob says more than sync limit (e.g. 20 messages). Alice comes back online again, and gets a gappy sync response. She then looks for Charly in the memberlist. (Should repro LL members can get out of sync with server on limited sync response  #7211)
• [ ] basic e2e encryption in LL room not doing this for now
• [ ] search for a username in the memberlist that was active before timeline we're aware off, also @mention ...?
• back-paginate and check correct display names are set in timeline

2 remaining things:

• creating rest users fails with 500 on CI server
• the gappy sync test showed a problem where the timeline isn't rendered when coming back online

[bwindels]

Adding users with:

cd installations/consent/
source env/bin/activate
./scripts/register_new_matrix_user -c homeserver.yaml -u charly -p testtest --regular-user  http://localhost:5005

[bwindels]

Also matrix-org/matrix-react-end-to-end-tests#16

[bwindels]

back paginating adds the /messages state to the timeline where it's appending events, which in case of a gappy sync might not be the live timeline, so it will not show the members it learned about in the memberlist. As synapse fixes this by returning all incremental state in case of a gappy sync, I won't fix this. Found this out while trying to repro the timeline not being rendered in the e2e tests. I assume that has something to do with scrolling all way up the timeline just before in the e2e tests...

[bwindels]

Indeed related to being scrolled up, see roomView.canResetTimeline(). Need to think if this is actually a problem and why we're doing it like that...

[bwindels]

It's not a LL issue in any case, so checking it off!

[bwindels]

Filed #7361 related to the gappy sync issue.