Remove double-leader mappings

Because earlier easymotion leader was remapped to non-default leader
Add sessions/* to .gitignore

Started to use Vim sessions locally
Remapped Git Commit View to ,,c

Because with one "leader" it waits for mappings from NERDCommenter
Restore autocompletion style
Release `,d` mapping & reserve it for NERDTree
Completely disable automatic GitGutter

Because it's slow. I run it manually on file save anyway.