Allow 's in mentions

When mentioning a user in a comment, or a similar place, sometimes one
would wish to use a possessive: `As per @user's suggestion` or somesuch.
This patch modifies the `mentionPattern` used to find mentions, to allow
- and then ignore - apostrophes and whatever comes after them.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

modules/references/references_test.go

@@ -392,6 +392,9 @@ func TestRegExp_mentionPattern(t *testing.T) {
 		{"@gitea,", "@gitea"},
 		{"@gitea;", "@gitea"},
 		{"@gitea/team1;", "@gitea/team1"},
+		{"@jess'", "@jess"},
+		{"@forgejo's", "@forgejo"},
+		{"Оно сломалось из-за коммитов от @jopik'а", "@jopik"},
 	}
 	falseTestCases := []string{
 		"@ 0",