Provide a function for validating translations

Summary:
Based on discussion at T1139#32, we will not provide context to translators.
Thus we should limit the translations to have the same special characters.

This disallows some originals: e.g. 'print <filename>' in text context - one should use 'print <%s>' with pht('filename').
And some translations: e.g. 'me & you' translated as 'me and you'.

We should call this from linter but I don't know how.
We can easily call it from addTranslations() in runtime but I'm not sure it's worth the overhead.

Test Plan: New test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D3164