Clang-format sees the text behind `#pragma region` as code and formats it. Instead of stating the copyright and date there, it's now in the comment block right below it. The text "Copyright" is left in the `#pragma region` line, as clang-format sees it as a single identifier.
I took the opportunity to normalize the dates, and add the copyright notice to the source files where it was missing them (except for third-party and the generated resources.h file).
BannerIndex is defined in common.h (like some other types such as money32 and rct_string_id), so that banner.h wouldn't have to be includede everywhere. Later on, it may be nicer to have a custom types file instead of putting them all in common.h.
At some places, BANNER_NULL and BANNER_INDEX_NULL were mixed, and at one point even MAX_BANNERS, causing the 250th banner to not be initialized properly. This commit makes the uses of those defines correct, and changes all relevant functions to return uint8 instead of sint32.
