Remove direct pointing to the exact font weight value

[malashkevich]

In lot's of places mapping has exact values to the fontWeight:

"textFontWeight": "bold",

however, mostly we are using variables:

"textFontWeight": "text-subtitle-2-font-weight",

Let's unify it and have the variables everywhere. This could help to easily solve issues like https://github.com/akveo/react-native-ui-kitten/issues/1399