Fix: Checking whether comment can be created

[jakubbortlik]

Hi @harrisoncramer, in this PR I address #424, by making the following changes:

1. All checks whether a comment can be created are now made for all three functions: create_comment, create_comment_suggestion and create_multiline_comment in a dedicated function gitlab.actions.comment.can_make_comment().
2. The checks are made in a more meaningful order (e.g., checking that cursor is in the reviewer is made before the check that current mode is visual mode for multiline comments).
3. The check for clean working tree before making comments is now correct and it's made on a per-file basis, i.e., unsaved/uncommitted changes in a different file do not block a comment in a good file.
4. Similarly, with a dirty working tree, choosing a MR is not blocked if that would not lead to switching the branch.
5. User-specified imply_local = true is overridden in the settings if the working tree is dirty. Before, it was possible that the setting imply_local was true, while the option was not actually applied to the DiffviewOpen command and so commenting was unnecessarily blocked for modified files.

There are some other improvements:

1. Visual mode is exited when multiline comment creation fails.
2. The ERROR level is used for all messages when a comment cannot be created.
3. A draft reply can now be created even if the discussion tree is "detached" from the reviewer.

I'll be grateful if you find the time to review and merge this.