Provide a proper API for updating translations

Problem/Motivation

Currently how we check for updated translations and then actually update the translations is baked into controllers and forms in the Interface translation module. This makes it hard to provide the functionality through non-HTML means, for example Drush. See https://github.com/drush-ops/drush/pull/1818

Proposed resolution

Provide a service for high-level translation update functions.

Remaining tasks

• Agree on the scope.
• Write tests.
• Write and check code.
• Write change record.

User interface changes

None.

API changes

A new service that wraps existing procedural code.

Data model changes

None.