feat: media-embedding in messages

As we can store arbitrary data in the git storage and reference it in a commit, we can support media in bug's comment.

In particular, we can have:

• images (jpg, png, gif ..)
• video (as supported by major browser)
• arbitrary files

Markdown support images but not video (though gitlab reuse the same syntax: https://gitlab.com/help/user/markdown#videos), nor arbitrary files.

On top of that, as files would not be in the normal repository, we need a way to reference the corresponding blob.

A way to do that might be:

Inline-style:
![alt text](e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 "logo.png")

Reference-style:
![alt text1][logo]

[logo]: e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 "logo.png"

Additionally, git-bug could enforce the reference style to make it more readable for terminal users.