Comment when branch has conflicts

[s4heid]

When the head of the pull request cannot be changed due to conflicts with the preferred branch, branch-switcher should send a friendly message reporting the problem.

Scenarios:

1. when head has conflicts with preferred branch
2. when head has conflicts with base branch

Expected behaviour:

change base branch to preferred branch and

1. comment on the pull request that after switching there are conflicts which must be resolved. It might also be helpful to mention the author.
2. comment with the standard message