Allow only one char for alias placeholder

[notramo]

In my opinion, it would look better if avatar placeholders would use only the first letter of the alias (not the first two).
Let's try it for a release, and if we don't like it, restore it in the next.

@0x1a8510f2 @EdGeraghty

[0x1a8510f2]

Currently pre-occupied so I won't be able to look at this, I'm afraid. Also, I am watching this repo so no need to mention me in issues, I get notifications anyway :D

[ereio]

Would like this to be toggle-able. I prefer 2, but others may prefer 1

It can default to 1, I don't mind here

[EdGeraghty]

Probably worth making it toggle-able; I'm ambivalent, but I can see how two letters over one would be useful as your contacts list increases. Also, it may also be worth looking over MSC3015: Room state personal overrides, which provides for overriding displaynames

[EdGeraghty]

One further observation: Signal shows either one or two initials, depending on whether the other side has a surname or not.

So Joe Bloggs will show JB, Joe just shows J.