Remove DM-guessing code

[lukebarnard1]

Should fix element-hq/element-web#3331

@dbkr and I decided that it would be easiest to just not be setting the DM state when rendering. I couldn't find any code path or theorise any accurate state of the app that could lead to a rendering of the RoomList being done before AccountData had been received.

[dbkr]

lgtm code-wise. @ara4n @lampholder is this OK by you? We figure this code is now not giving anyone any benefit since nobody will be upgrading from old versions anymore, and it will prevent the disappearing people section bug from being a problem.

[ara4n]

i thought we were going to fix this by having getAccountData() distinguish between "empty" and "not yet known"? I can see the upgrading behaviour being quite useful for people migrating to Riot from other matrix clients, etc...

[lukebarnard1]

@ara4n there were a few complications with this, namely that having traced through the state that the client has to be in for the RoomList to appear, the client has done a sync by that point. So neither @dbkr nor I can figure out why it would ever appear without the true account data being cached. That is unless the server did a weird thing and sent account data in a later sync (breaking the assumption that first sync = accurate representation of server state).

Also, in trying to add sync state checks to the relevant places to prevent getAccountData() from being called without a sync having been done, I found that the only reason it would be called would be a js-sdk event being fired, causing one of the views' onAccountData to be triggered. The only reason for this firing is if the sync response is being processed, in which case there's no point in checking.

[lukebarnard1]

As suggested by @ara4n, it would be a Good Idea to have a "Guess DMs" button in settings so that users moving over from a client that doesn't deal with DMs or users that have previously lost their DM maps can reset them.

[lukebarnard1]

I'm going to merge this to avoid more DM sections going missing and make a new issue to track the "Guess DMs" button