When the text says "here are two spaces to force a line break"
there really should be two spaces to force a line break.

Group milestones can only be referred to by name, not IID. They also do not
support cross-project references.

Also adds details/summary tags to Copy-as-GFM

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Signed-off-by: NRémy Coutable <remy@rymai.me>

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`.ogg` is for music files.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Handle videos in:
- MD preview in notes: commit, issue/MR, MR diff
- New notes in: commit, issue/MR, MR diff
- Persisted notes in: commit, issue/MR, MR diff
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Signed-off-by: NRémy Coutable <remy@rymai.me>

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