Track review edits as separate actions

[mathjazz]

Rather than contact fellow localizers about changes they need to make to a translation, reviewers often make changes themselves. This type of edits should be detected as an action (e.g. TRANSLATION_EDITED), separate from the action of submitting a new translation (TRANSLATION_CREATED), and stored accordingly in the ActionLog.

This change will allow us to track activity and attributions more accurately, for instance in the following places:

• Add Timeline tab to Project, Locale, ProjectLocale dashboards #2138
• Improving profile page #2028

This issue was originally submitted as part of #2062.

[eemeli]

As noted in #2471, once this is implemented, the relevant notification ought to also be updated.