panettone: allow inline formatting (e.g. code markers) in subjects

A subset of inline formatting should be possible in subjects.

• strikethough
• italics
• code

1. https://cl.tvl.fyi/c/depot/+/2389/

   sterni at 2021-01-14T02·01+00

2. sterni closed this issue at 2021-01-29T22·54+00

3. We now support non nested strikethrough, italics and code.

   sterni at 2021-01-29T22·55+00