* make mission log dynamic
Change the mission log from a static array to a dynamic vector. This removes any chance of the mission log running into a limit (as regularly happened with the last mission of Shepherds before the limit bump) and also prevents strange mission bugs caused by culling obsolete entries.
This also unifies the API a bit by renaming the public `message_log_` function prefixes to `mission_log_`, removing the redundant `Num_log_lines` variable, and removing `Log_scrollback_vec` from the header.
Implements #5878.
* feedback
* refactor to reference rather than pointer