Add ability to manually pretranslate strings #2790

Teams might want to benefit from pretranslation capabilities on projects for which pretranslation is not enabled (by the admins).

Manual pretranslation would be triggered by Translators, either for the entire project or for a subset of strings. We could also add it as an additional batch action.

We should take into account that pretranslation is a background process that can take some time and modify the UI accordingly while it's in progress.