Rename Default font to Override font to describe functionality

I added res strings to a res/values file, but inserted nothing into the res/values-XX files for foreign language translations. I assume a process exists so that those are somehow generated from that master, and translated along the way through crowdIn, so if I actually need to do something else to complete this change, please let me know.

No functionality has been changed. New res strings have been added to describe the current functionality, and the old res strings have been left for the implementation that is worded (that of default font which we had be reverted to an override font). All variables and functions have been renamed to reference override in place of default.

I chose to say that an override font replaces any fonts specified "by a deck", knowing that fonts are not at all deck specific (living in the note/card instead), but I felt the description given lends itself more intuitively to an average user, which is fitting since this override functionality is expected to be used by those who rarely if ever work with cards or CSS but want a general feel even across shared decks.

This is a follow up to a change by @hssm that returned from a default implementation back to a user desired override implementation that had been around.