HACKING.rst and import standards

It appears that rebasing on changed master branch is not a trivial task now. When nearly each Horizon module includes comma separated imports, all patch sets including import from the same module in an existing import statement can't be merged automatically. You have to rebase manually, just because of a few imports.

It would be nice to implement all imports uniformly like that is done in other projects. For example we could write one import per line or just replace all related imports with module import. That would solve merging problems and code would look much better.

It also would be nice to add HACKING.rst to Horizon. We could describe coding standards there and we would (hopefully) never get such kind of rebase and merge problems.

Since we start using HACKING.rst, the following import standarts are to be enabled:
F841 local variable '<smth>' is assigned to but never used
H101 Use TODO(NAME)
H201 no 'except:' at least use 'except Exception:'
H301 one import per line
H302 import only modules
H303 No wildcard (*) import
H304 No relative imports
H306 imports not in alphabetical order