Make it easier to get people chatting (guest access)

(This issue isn't really just about riot-web. It's really about the whole ecosystem, but I need a place to write it down and this will do…)

I've gotten several dozens of people to join Matrix in the last couple of weeks, and found the process unnecessarily complex. Often, I'd have to wait for the person to install the app so that I could then let them know how to connect with me, or join the conference chat room. Per se, that wasn't so bad as I used to time to explain some concepts around Matrix, but quite a few times we simply ran out of time, which hindered adoption. Furthermore, I found a lot of this to be repetitive and error-prone, thus calling for automation.

In an ideal world, either of the two should be possible:

- Hand someone a business card with a QR-code or the like. Scanning this QR-code should then leave the correspondent in a direct chat with me, whilst my e2e-encryption key is already verified.
- At a conference, the organisers might want to provide a QR-code that gets people joined into the conference chat room.

Obviously, this can go to any level of detail, but maybe the following could be a viable way forward?

1. The QR-code links to a unique page e.g. on `matrix.to`. On that page, instructions on the available clients and their download links etc. can be provided;
1. Maybe it could feature a simple means to sign up for a new account, i.e. I enter my desired name on `matrix.org` (or maybe other public servers could be listed too as options) and if it's available, a data file gets generated that can be fed to the client, who would then automate the signup process and essentially only prompt for a password when it's time.
1. Finally, a data file gets generated that if fed to a client causes the client to auto-join a direct chat with me, or a conference room, or any combination of these. This data file could also include information necessary to verify e2e-keys, as applicable.

Probably the two data files in (2.) and (3.) should be combined into one and then fed into the app at once, so as to simplify control flow for the user.

With the Riot web client, no data file needs to be downloaded. A link to the webapp could encode all this information, or at least the URI where to download all these data. The mobile clients probably need to register themselves as a MIME handler for this type of file such that they get invoked to handle the download.