Ellipsis: Set msg nick indent + ellipsis based on user nick
The length of the users nick now controls the ellipsis and total indent of nick-to-message in the message display buffer.
- user nick update redraws current buffer to adjust to new length+indent
- this only happens if the user changes their nick
- user entry box indent set by msg display indent
- final indent set by max between users nick width or average char width multiplied by min char count (currently 10)
Addresses long-standing bugs bgo#762078 and bgo#765229