Gitignore: "Convenience for devs" section

MantisBT devs frequently have to switch between older and newer
branches. Some files and dirs need to be excluded for convenience, and
also to prevent changes from being inadvertently checked in.

Regroup various file exclusions in a single "section" at the end of the
.gitignore file, to facilitate maintenance:

• v1.2.x config files (moved)
• REST API (new)
• Composer's 'vendor' directory (new)

Backport from master-2.2 branch.
Conflicts:
.gitignore