Iterate on the UI / UX of the slack alert

• Links follow a format similar to the one used when unfurling URLs: title + namespace
• Links are no longer backticked
• Issues are more concise
  • no more labels or created at
  • a simple emoji when the vuln tag is present
• Sort is now by updated_at (so least recently updated MRs are alerted first). Shouldn't make a big deal in practice since we run every 10 min.
• We handle special chars (like <) in issue / MR titles

Screenshots

Before	After

Closes #16 (closed)