models: Add UserBaseSettings class to include some user settings.
models: Add UserBaseSettings class to include some user settings.

We add a new class UserBaseSettings and will be moving some of
the user settings to this class from UserProfile and UserProfile
will inherit it.

This is a prep commit for adding RealmUserDefault table which will
be used to set the realm-wide default for user settings like night
mode, etc. Adding UserBaseSettings will help us in avoiding copy
the same fields in RealmUserDefault.

andersk/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,punchagan/zulip,rht/zulip,andersk/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip