Add/Change: Adaptive server name in prompt for ement-create-room

[rayes0]

Quick change to show the homeserver for the selected session instead of matrix.org when creating rooms.

[alphapapa]

I appreciate this idea. It might be worth doing. But first:

1. The copyright assignment still needs to be done. (Technically you could contribute up to 15 lines of changes to the project without doing so, but I don't want to have to keep track of that sort of thing.)
2. The indentation in this patch is very inconsistent. Please use (setf indent-tabs-mode nil) and be sure to indent code properly before submitting (I recommend using aggressive-indent-mode for Lisp code, as it completely solves this problem).

[alphapapa]

Well, I do appreciate your fixing the whitespace, but you also changed the whitespace in the entire file rather than just the lines you changed. (Having worked on Ement in a different Emacs config for a while, which I neglected to set indent-tabs-mode in, it still has some tabs in it--I'll need to fix that in a separate commit sometime.) Please update the patch to only have the lines you've added.

[rayes0]

Should be good now, didn't realize you didn't me to fix the whole file :)

[alphapapa]

Thank you. :)

[alphapapa]

I think we can just use ement-server-name here, right?