Custom default messages implementation for single and multi-user modes (#193)

* added ui for custom welcome messages and added label for custom logo in admin settings

* linting

* fixing img to use light/dark modes

* converted ChatBubble into component

* implemented backend for welcome messages and admin appearance page

* completed custom welcome messages for admin

* finished custom messages for single user mode

* merged with master and linted

* improved UI for appearance settings pages

* linted and merged with master

* small updates

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Co-authored-by: timothycarambat <rambat1010@gmail.com>