Text size as a domain setting

[gkrishnaks]

Is your feature request related to a problem? Please describe.
Currently in Fulguris, Text size is a global setting. It would be very useful to have text size as a domain setting.

Describe the solution you'd like
Solution : Add Text Size into domain settings. Existing domain settings already supports the flow of disabling a global setting and apply a domain specific setting. Same can be reused for text size per domain or else use global text size as default for a domain.

Describe alternatives you've considered
No alternative. Currently Text size is a global setting for all domains.

Additional context
Sites use different styles/fonts and so a global setting may not be enough. It would be very useful to have Text size as a domain setting.

[Slion]

Related to #159