Milestone 2ia Consolidate CSS of chat pane

[timea-solid]

Chat-pane needs to be updated. This ticket is for the CSS part.

TODO:

• extracting all inline CSS in dedicated CSS files (example in profile-pane) Very important is to create dedicated CSS pane names. Example: if you use the .warning, its CSS will be coming from mash.css so create a dedicated .warning-contact for it if the one in mash.css does not fit.
• making sure the CSS is part of the release. Maybe we need a webpack) build, and an accompanying entry file (example index.ts)
• using already available CSS variables from mashlib light theme and documenting in the PR what other CSS variables could be moved to the main mashlib theme
• fix some issues or implement some features expected of chat to make it better. (up to you to decide which)