Use route helpers to construct URLs in ChatMessage module

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Description

In various classes in ChatMessage module we manually construct URLs. Example (from ChatMessage::PushMessage):

def branch_url
  "#{project_url}/commits/#{ref}"
end

This has a big disadvantage: when our routes change these URLs will potentially break

Proposal

Use Rails route helpers to have a single source of truth.

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8641/diffs#note_21949047