Rename donation(s) to tip(s)

[NejcZdovc]

When we start rewards project we wanted to add donations. But at the later stage this name was changed to tips. Because of that we have both usage in the code. To unify the code we would like to rename all donation(s) mentions into tip(s). There was already first step made into this direction with this issue #3953. Second step(this issue) needs to be more aggressive where we need to rename files and variables. Next step (follow up to this issue) is migrate state values to new naming convention. Last step is to update brave-ui repo so that we remove donation mention from there as well.

For this issue:

• rename all files that contain donation phrase to tip
• find all variables, class names, definitions, etc. that contain donation phrase and rename it to tip