Users should be able to filter out bot comments on merge requests

Problem

Currently user-users and bot-users display similarly in merge request discussions, with the notable exception that bot comments can't be minimised by "resolving" the thread, like user comments. This means that in setups with either excessive amounts of bots, or huge bot comments, these bot comments make the discussion thread extremely long and frustrating to find user feedback in.

We already offer a filtering system for the discussions list that allows users to remove everything but comments, but this still includes the bot comments.

Proposal

Add an additional option to the filters dropdown that allows a user to filter out comments from bot users. We could split the current Comments option into User Comments and Bot Comments, or maintain Comments and add a new Bot Comments option.

When Bot Comments is not selected, no comments made by bot users should be displayed.

Availability and Testing

Add feature specs for filtering bot comments in an MR. Ensure bot comments are displayed when not filtered.