R1:7189cb7ba816

Support text encoding and syntax highlighting options in document rendering

Summary: Depends on D19273. Ref T13105. Adds "Change Text Encoding..." and "Highlight As..." options when rendering documents, and makes an effort to automatically detect and handle text encoding.

Test Plan:
- Uploaded a Shift-JIS file, saw it auto-detect as Shift-JIS.
- Converted files between encodings.
- Highlighted various things as "Rainbow", etc.

Maniphest Tasks: T13105

Differential Revision: https://secure.