feat(webapp): change chat style and lang

Created by: maeckes-infinitylabs

Pullrequest

• PR adds text snippets of the chat component to the i18n module for translation.

• It adds the option to customize the chat style via the base token SCSS variables.

• Customized "menu-icon" slot to have close icon for singleroom chats

• Customized "room-header-avatar" & "room-list-avatar_" slot to to handle the case if no avatar-image is available

Issues

• relates #6400 (closed)
• relates #6533 (closed)

Todo

• None