User pills get in the way when adding new trusted reporters

**Describe the bug**
When adding trusted reporters and auto-completing their name with some clients (such as Element), their name is turned into a pill. Mjolnir accepts it. It doesn't understand it as a matrix ID though.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to the moderation room, from Element
2. Enter `!mjolnir config add TrustedReporters.mxids `
3. Type the beginning of the name of someone who is in the moderation room, and press tab
4. Press enter. Mjlonir will accept the input.

**Expected behavior**

Mjolnir should either be able to find the matrix ID from the user pill, or warn that the trusted reporter is not a Matrix ID (and thus cannot be used).

**Desktop (please complete the following information):**
- OS: macOS
- Version: Nightly 2022072901

Start by tracing the `!mjolnir config add TrustedReporters.mxids` command and how an Element user pill is represented when received. Reproduce the flow from the issue, then verify that a pill is either resolved to a Matrix ID or rejected with a warning rather than accepted as a trusted reporter.

Written by the indexing model from the issue text.