Make color for edited messages configurable

[IngoHohmann]

The green for the current version of edited messages in gitter rooms is too light on my android device with current riot android, and barely readable.
Yannick looked it up, and says that the colors are defined in the message.
So I would like to have colors in there, which are configurable.

[leonerd]

Configurable at what level though? Is it global for the entire bridge? Per channel? Per user?

[IngoHohmann]

I'd say per user.

Maybe it would be OK to just change the color entirly to something darker, but I seem to be the only having this problem, so far.
Interestingly it looks fine in screenshots, but on the device itself it's barely readable for me.

Also somewhat related to Issue #28

[avalenn]

I think this issue should be closed in favour of #78