Polish conversation view

I'd like to improve consistency between the different views before the release.
The submit button on the conversation view is different than the others and at left instead of right.

The style for the buttons are defined in many different places (in buttons.css but also [in the conversation file](https://github.com/diaspora/diaspora/blob/develop/app/assets/stylesheets/conversations.css.scss#L235) or [in the SPV file](https://github.com/diaspora/diaspora/blob/develop/app/assets/stylesheets/single-post-view.css.scss#L236), I guess also somewhere else for the stream...

I didn't follow exactly the refactor of the stylesheet files, in which file should I define the style of this button, once for all?