Related to the discussion at #532.

Wakapi intentionally ignores the <<LAST_PROJECT>> placeholder, so to attribute browsing activity to a project, you'll have to use the @@project syntax in the browser plugin's whitelist setting. This will then result in an actual project being set on the heartbeat.

If a heartbeat's project field is set and <<LAST_BRANCH>> is set as a branch, resolve that placeholder to the last used branch on that project. If both fields are set to their respective placeholder, keep ignoring them both.