Mission protocol refactoring

I was having issues with the mission manager not properly communication with my autopilot. I started digging into the code thinking it was a QGC issue, and since I didn't understand the code quickly, ended up refactoring it to make it easier to read and add comments.

Did fix a bug where the `target_component` wasn't checked for `MISSION_COUNT`, `MISSION_REQUEST`, and `MISSION_ITEM` messages.

Also moved some more strings into `tr()` translation function calls.