Refactor: `*Helper` classes as companion classes to `*Controller`

All additional repository specific algorithms should be bundled in classes that
are named `*Helper`. They should be companions to the `*Controller` classes
that access the repositories over the internet.

For example `BugLpFilter` and `buglp_equal_contents` should be converted to
`LpHelper`.

Collecting the repository specific algorithms in one name space becomes more
interesting, when an algorithm is implemented to conserve status information
better. (For example bug #949820, bug #959770.) This algorithm would be one more
repository specific function that must be implemented for each repository.

'''Related:'''
 * bug #949820 (Conserve Launchpad's bug status values)
 * bug #959770 (Represent Bug Status Values as Sets)