client: Migrate old highlight Channels to RegEx

client: Migrate old highlight Channels to RegEx

Add new settings minor version 8 to migrate existing local highlight
rules to the new format.  Specifically, any highlight rule with a
specified "Channel" filter is adapted into a regular-expression rule.

> Before
Name: a word
RegEx: disabled
Channel: (ChannelA|AnotherPlace)
> After
Name: (^|\W)a word(\W|$)
RegEx: enabled
Channel: (ChannelA|AnotherPlace)

This way RegEx channel rules will work by default now that the RegEx
flag controls wildcard vs. reg-ex for channel names, too.  And this
specific migration is backwards-compatible with previous client
versions, so downgrading won't break highlight rules.

(Functionality changed in other migration steps might break on
 downgrade, however.)