Allow arbitrary events to be assigned as "similar" instead of exclusively relying on Sentry's determination

### Problem Statement

While I acknowledge that the "Similar Issues" feature is experimental, there are simply going to be events that Sentry can/will not identify as similar that actually are, and the current implementation of the feature leaves Sentry power users without a way to handle the similarity themselves.

### Solution Brainstorm

Allow Sentry users to identify and self-assess what events are similar by adding a way to specify an event as similar in the UI.

### Product Area

Issues

The issue concerns the experimental Similar Issues feature in Sentry's Issues product area, but it names no files, tests, or entry points. Start by locating the Similar Issues UI and its current similarity-assignment flow; done would mean users can specify an event as similar themselves.

Written by the indexing model from the issue text.