Server-side translation UI

This is implementation work for https://github.com/secondlife/roadmap/issues/141

SL-20168 branch has an initial prototype. To use:

• Run a viewer build from the SL-20168 branch
• Go to a supported region on aditi. Currently the vir-dev channel - regions VirCloud and SST - have support.
• In preferences, set the target language. Do not check the translation box, as this is only used for client-side translation
• In nearby chat or IM, right click on the name of someone who is communicating with you (or on yourself). In the context menu, pick "Translate Chat".
• Chat from the selected person will be translated into the selected language.
• To turn translation off for the person, right click on their name in chat again, and pick "Don't Translate".
• The settings for who to translate are currently stored only for the duration of the session. After you log out, or if you connect from another machine, you will have to re-select to translate them.

Note: if you do select viewer-side translation by clicking the box and configuring a translation API, this will take precedence over server-side translation.