Support suggesting in zen mode

**Is your feature request related to a problem? If so, please describe.**
I noticed that a user with only "Suggest" privilege can not use zen mode.
The user has no zen mode button on the translation interface. And if he manually enters zen mode by changing URL, weblate will alert "Insufficient privileges for saving translations.".

**Describe the solution you'd like**
I wonder is it possible to allow a user with only "Suggest" privilege to contribute Suggestions using zen mode. The zen mode does help boosting efficiency for some users. So it's frustrating that they can't use it.

**Describe alternatives you've considered**
I'm going to do some frontend hack to see if my idea is possible/easy to use.

**Additional context**
If a user with both "Suggest" and "Translate" privilege is using zen mode, it might be confusing whether he is going to submit the contribution as Suggestion or Translation. My idea is providing a single select option somewhere, which can control if it's a Suggestion or Translation. The option should default to Translation if the user has "Translate" privilege, and disabled(greyed out) the "Translation" option if the user doesn't have the privilege.