Review and revise accessibility methods in postflight mini-DSL

* rename `remove_accessibility_access` to `disable_accessibility_access` to match corresponding method `enable_accessibility_access`
* move `TCC.db` and `.AccessibilityAPIEnabled` paths to `Cask::Locations`
* remove unneeded backslash from `TCC.db` path
* use full paths to `touch` and `sqlite3` utilities
* reverse conditional so enable/disable logic is consistent
* `sudo` is needed when creating `.AccessibilityAPIEnabled`
* instead of silent fail, warn that access cannot be safely disabled on Mountain Lion and earlier (it could, but might affect other apps)
* quoting/whitespace

Todo: the relevant methods are only available in either `postflight` or `preflight`, but they should be in both.

Edit:

 * use `@command.run` instead of `system_command` which is intended as an  external interface

cc @federicobond