Adjusted user mention parsing, added tests, and parsing for attachment fallback.

[coltoneshaw]

Summary

When importing from slack I noticed <!here> was not properly converted at all. This adjusted the regex to find both <@here> and <!here>

Additionally, added parsing for attachment fallbacks because they use the mention syntax.

Before the change:

test <!here> test
test @channel test
test @all test
test @user1 test
test <@user1> test

Before the change:

test @here test
test @channel test
test @all test
test @user1 test
test <@user1> test

[coltoneshaw]

I don't believe <@user1> is the correct parsing here and I'm happy to adjust. I'm just not sure what we were looking for in slack, so I haven't touched it. https://community.mattermost.com/private-core/pl/6wd3kptwmfnhzdgqeb9a4wtcuh

[mickmister]

Not sure I understand this line. Doesn't this effectively do the following?

Suggested change

	attachment.Fallback = r.ReplaceAllString(attachment.Fallback, mention)
	attachment.Fallback = mention

[coltoneshaw]

if so, wouldn't we modify the above string too?

post.Text = r.ReplaceAllString(post.Text, mention)

It's just passing the mention regex into replace

[mickmister]

I understand now. I didn't realize r was its own regex