Multiproject preferences updates

[stwalkerster]

language and default domain option in preferences, split prefs into new table

• create table domainprefs....
• alter table user add column language varchar, add column defaultdomain int
• migration script to move preferences to domainprefs table
• drop columns in user table
• adjust prefs page to use new preference locations

domainprefs

This table provides preferences for a domain, and is distinct from userdomain which only holds whether a user is able to access a domain or not. Notably, rows in this table remain even if a user is removed from a domain.

Name	Type
id	int autoinc
updateversion	int
user	int not null fk
domain	int not null fk
creationmode	int not null
welcomesig	string
welcometemplate	int fk
emailsig	string

[stwalkerster]

Based on the idea posted in #675, the approach taken here is:

• Remove welcome_sig as unused.
• Remove welcome_template, abortpref, emailsig, creationmode, and skin from the user table
• Create new userpreference table as (user, domain, preference, value)
• Migrate removed columns from user into rows on userpreference
• Built a new preference manager class to handle the difference between global and local, and arranging fallbacks between them as needed
• Migrated everything to use the new architecture.

Domain specific settings get the domain tagged against them. Global settings get the domain column null. There's more complexity here surrounding the global generated column and the constraints to make this behave properly, but I consider that an implementation detail.

I'll leave the language preference out until we sort out internationalisation.