Enforce reviewer policy on Weblate

Originally created by @intrigeri on #16855 (Redmine)

I.e. enforce “one needs to have Reviewer status to be allowed to accept suggestions” and make sure that non-admin users can’t change this setting per-component.

Blueprint: https://tails.boum.org/blueprint/translation_platform/#access-control

Related issues

Related to #16881 (closed)
Related to #16989 (closed)
Blocks tails#16326 (closed)

Edited May 15, 2020 by intrigeri

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