Improve UX for concurrent translation of a single language

Hi,

We use weblate for translating [CryptPad](https://github.com/xwiki-labs/cryptpad) and we're very happy with it, so thanks!

It seems that when two or more editors are translating the same language concurrently the same translation keys are suggested at the same time.

For example, if there are ten untranslated keys in French and two people click the "translate" button from the project's language list, the server will suggest that they edit the same keys in order. They may each take several minutes to translate before submitting, at which point one of their translations will override the other (presumably the most recent submission will "win").

In practice this can be addressed by coordinating with other translators over some other medium like a chat room, but that makes the platform a little clumsier to use as a group.

One alternative way of handling this would be to randomize the order of the next chosen edit out of the list, such that if Alice and Bob start editing ten strings at the same time, Alice will start working on string number five while Bob starts working on number nine. There will still be a chance that they work on the same translation at once, but it will be relatively unlikely unless there are a large number of translators and/or a small number of strings remaining to be translated.

Another alternative would be a locking system in which one editor is told that somebody else is already working on a string. This would eliminate the probabilistic nature of the random approach, but would also pose UX issues if somebody kept a tab open for a long time. It also seems like it would be more complex to implement.

Please let me know if there's anything I can clarify and thanks for your excellent tool!