client: Deprecate local highlight, migrate cleanup

Deprecate "Local Highlights" for client and core setups, adapting the
name "Legacy Highlights" as used with monolithic installs.  Make the
legacy nature more prominent, but keep support enabled.  In the
future, legacy highlights should be disabled and invisible except when
connected to older cores.

Rename "Remote Highlights" to "Highlights" and place first in the
settings list.

When using the "Import Legacy" button, migrate nickname highlight
settings and offer to remove the old local highlight rules, avoiding
redundant highlight rules.

This should help reduce the cases where Highlight Ignore Rules are
configured, but don't seem to respond due to client highlights
overriding the core-side highlight ignore rules.