Get rid of hardcoded font sizes

[sronveaux]

As stated by @fschmenger in #463:

I noticed we're making use of some hardcoded font-sizes across the projects css, e.g. in AppFooter, Map, AttributeTable. This wont go very well with customizable fonts. Maybe we should open up a follow up issue to keep track of some remaining tasks.

Ideally, hardcoded font sizes should be replaced by Vuetify text classes when applicable or with some similar custom classes making use of relative units in most specific cases.