Get rid of whitelist/blacklist getter code duplication

[vergeev]

Lines like this repeat over and over again.

Since we need a way to protect ourselves from regressions, this ticket should be completed first.

[vergeev]

Current problems:

• the whitelist names are implicitly chosen
• the whitelists are far away from the place they are being used in
• under current convention of naming whitelists, there can be no semantically different whitelists simultaneously (e.g. a whitelist of files and a whitelist of variable names)

[vergeev]

The problems were addressed in the following ways:

• Whitelist names are actually chosen explicitly by the validators on the lines such as:

whitelist = whitelists.get('has_no_extra_dockstrings_whitelist', [])

• Whitelists are far away, and his is a good thing, as it allows the clients to parameterize the lists. We could just write a decorator with skip_dirs parameter, but this would not allow the clients to parameterize skip_dirs easily.
• This is being addressed in Improve the names of whitelists and blacklists #102

Finally, in PR #100 we allowed to skip user-chosen directories (by all validators and pre-validators at once. We made "a global whitelist" basically):

• 5509db4
• 7804203
• e8600cd