Inline links for common string actions

[nbachiyski]

Currently: some actions are doable only via the Bulk menu, which is very inconvenient if we need to do it on only one string.

Here are the actions we need:

• Approve
• Reject
• Translate via Google Translate
• Mark as fuzzy/not-so-fuzzy

(In [529]) Inline approve and reject buttons. Fixes #96

• Approve button shows only for strings with translations, which are not approved
• Reject button shows only for strings with translations, which are not rejected
• Route methods, which edit a single translation and return an updated one via ajax were consolidated and use a common private method
• GP_Translation::set_status() was introduced to replace GP_Translation::set_as_current() and GP_Translation::reject()
• Approve/reject buttons and priority dropdown were removed from tab order

A fuzzy button wasn't included, because fuzzy status should be set only automatically.